I also feel that infantry are a fine place to start because there's really no getting around them. Every army needs at least a little infantry in order to claim objectives. Note this movement also applies to monstrous creatures that do not have wings. I do not believe there are many Fleet monstrous creatures (The Daemons have the Keeper of Secrets and Skarbrand, and that's all I've got for fleet MCs), but there ARE some that are oversized jump infantry.
Warhammer 40k is a tabletop war game. Given terrain features, there are three dimensions to the game: up/down the table, left/right on the table, and up, on terrain (or skimmer stands, or the Valkyrie/Vendetta stand, or on top of burnt-out vehicles). I'm going to touch on the more 2-dimensional movement, which is on the tabletop. Note that moving through terrain to get atop things takes 3" of movement, so you'll lose some horizontal range in order to get atop a building.
Basic Movement in the OpenInfantry can move 6" by definition. Infantry may also run 1d6 inches, which translates to about 3.5 inches per run. This picture demonstrates the 'average' infantry movement in the open, though note that the actual movement radius is 7-12 inches, depending on your run roll.
Basic Infantry and the AssaultInfantry may also assault 6", so an infantry's assault range is a total of 12". Assuming a victory in assault, there may be additional movement in the form of consolidation. Note that an infantry assault can move the unit up to 12" in a turn, and an additional 1d6 if you beat the other guy's face in.
Fleet Infantry and Assault
Infantry with Fleet of Foot differ from regular infantry only in the assault phase, with regards to movement. Fleet infantry may run, then assault. This means a Fleet unit's assault range is 13-18 inches. Now we're talking some movement, right? Note, though, that the average is about 3.5 inches in the assault, so a fleet unit's average assault range in the open is 15.5 inches. Not bad? Wait 'til we discuss cavalry.
Infantry and Terrain
Terrain is infantry's friend: lurking in terrain grants you that 4+ cover save you always wanted. Guardsmen, for example, prefer terrain to their armor. However, terrain slows you down, as you must roll 2d6 and take the higher instead of assume you'll get 6 inches.
Odds of a given outcome:
1 inch: 2.7%
2 inches: 8.3%
3 inches: 13.8%
4 inches: 19.4%
5 inches: 25%
6 inches: 30.5%
4-5 inches is the norm for a terrain move. Note that the actual range is 1-6 inches, and there's nothing like rolling snake-eyes when you actually needed to be somewhere.
This also applies to Slow and Purposeful infantry; Obliterators are a prime example.
Terrain and Movement As a Whole
You can run in order to maximize your movement through terrain, which gives you a range of 2-12 inches. This will usually be around 6 to 8 inches total, and if you are fleet and assaulting through terrain, then you'll have a range of 3-18", or around 9 inches as a rough average.
The morale of the story? Well, first, bring grenades if you want to assault through terrain (terminators with powerfists/thunder hammers are exempt...). Second, Terrain slows you the hell down.
Note that Monstrous Creatures benefit from Move Through Cover, so they are more likely to see a 5 or 6 on the 3d6.
Movement and Shooting
The basic infantry weapon is the 24" rapid-fire gun. If you want to move and shoot, you get your regular move (6 inches in the open, or about 3-4 with cover) and then a 12" double-tap. In the open, this means your effective range is 18", and with cover your effective double-tap range is 13-18, or an average of 15-16 inches.
A Warning on Rapid-Fire
12" is also the magic number for an assault in the open. If you are in cover and rapid-firing out of it, then the enemy will probably get a 6" move, and then another 3-4" into terrain, which may or may not be enough to get them there.
In other words, if you can rapid-fire into the enemy, he can probably punch you next turn (unless you kill him). Moral of the story? If there's going to be a fistfight after you rapid-fire the other guy, make sure you can WIN the fistfight. Note also that you cannot assault after a double-tap, so you WILL be on the recieving end of any charge more often than not.
A Note on Assault Weapons and Pistols
These weapons may fire even if you have moved, so use the maximum range of the assault weapon in lieu of 12" in your calculation. Pistols usually have a 12" range, though some melta pistols have a 6" range.
Shooting without Moving
Stationary rapid-fire weapons may fire once at their max range (24" unless you're a Fire Warrior or Krootox), or twice at 12". The benefit to a stationary close-range rapid-fire is that any troops who are 13-24" away still get a single shot. Or, you just might not have anything better to do with the guys. Rapid-fire guns at max range usually don't put out a lot of shots, so their ability to do damage is somewhat diminished. As with close-range rapid-fire, Imperial Guard squads with 'First Rank Fire, Second Rank Fire!' are a notable exception since they get +1 shot for each lasgun.
On Infantry-Born Heavy Weapons
Stationary firing ALSO permits you to fire heavy weapons to their full range. Some infantry, such as Terminators or Obliterators, may fire heavy weapons even if they have moved.
Flow and the Game
These pictures cover a single turn of action for infantry. What can we learn from these diagrams? Infantry are not exactly mobile. The best you'll get out of Fleet infantry is an 18" move, IF there is something to assault at the end of the road. More often than not, infantry are slogging 3-6 inches a turn, and up to around 9 on average if we want to run them and not use their guns. Sometimes, you don't need them to shoot; last-minute objective grabs come to mind.
Some Numbers for your edification
Infantry in the Open Through Five Turns of Movement: 30" + 5d6, or about 47.5 inches average maximum. Not bad, until you consider that you have to do nothing but move and run all game, avoiding terrain (which may mean taking a winding path, mind you).
Infantry in Cover Through Five Turns of Movement: 5 turns of ~4 through cover, and 5d6 gets you 37.5 inches. Again, all you're doing is running, but at least you're in cover.
What's this tell us? Infantry are slow. If you're playing table quarters, for example, I can slap my objective in the back corner. If you start infantry as close as you can, you're still looking at a good 51-55 inches to get to the objective. In other words, you just may not have enough time in the game to get there, even IF all you do is move. Now, if you luck out against an enemy that's playing Pitched Battle, you might have as little as 24"-36" between you and the objective, but it depends.
Other Problems with Infantry in the Open
There's this little thing called 'shooting'. It may or may not be a problem for you. If you are Terminators and the other guy didn't bring a lot of AP2 guns, you might be fine. Or, you might have just brought a LOT of Orks/Guardsmen/'nids. Otherwise, you're probably looking at an uphill game. The big problem with moving a lot of infantry is, of course, time.
Dealing with the Speed of Infantry
As I have shown, basic infantry are just not that fast. So, what do we do about getting them where we need to go?
1) Mount Up
Vehicles are faster than basic infantry, generally speaking. Infantry in a transport are more survivable (since you have to get them OUT of it first) and of course the thing can move, barring an immobilized result.
2) Don't Move Much
Infantry camping on objectives (ideally with long-ranged weapons) don't need to move to contribute to the battle, for example. The counterassault element behind a static gunline only needs to be able to move out and punch someone in the face. Sometimes, the relative immobility is moot; you either don't need to go far or you have the range to reach out and touch someone.
As an added bonus, some stuff just gets nastier as you close with it. That lone heavy weapon is suddenly a heavy weapon with a bunch of rapid-firing friends, now.
3) Careful Planning
Foot armies that advance on you require careful advance planning. I have to point to my experience with the Black Templar list I'm running. Deployment makes or breaks you, as you WILL have to walk to the objectives. Your foot troops will be vulnerable to gunfire, and in some cases morale checks. Foot troops may also be corraled by things like assaults, tank shocks, and so on.
Foot Troop Flow Conclusion
My aim with the diagrams here is to give people an aid and example of what your footsloggers can pull off in the space of a turn. The real goal to learn via experience is chaining several turns of action together, and figuring out where that might leave you. Is it easy? No. My hope is that I've offered people some tools and/or food for thought for noticing the flow of battle.
Knowing how units may act during a single turn should give you some insight into how units can act over the course of a game.
Comments and Feedback welcome.
*As usual, thanks to readers; math credit goes to hastur first. TOO SLOW, DVERNING! Though, I completely agree with you on the infantry horde problem. It's realyl the only downside I can come up for something like pure foot guard; they'll shoot the crap out of you, and in a tourney setting you'll get ~3 turns, maybe four, just because of the setup.
**Regarding pictures, I think a radius probably IS the way to go. The problem is that without scaling it somehow, it's easy to lose definition because I'm looking at a 48" diameter for infantry stationary rapid-fire and fast vehicle movement. I think what I'll do next is try to show a sample of several turns and what they can look like for an individual unit.