Well i dont actually build units too early on in the game.. Well i do build kind of big army early on if im playing Romans (because they have Legion and Ballista

) but i have not enountered this problem you describe. I can have lots of fun with my Legions and Ballistas before they become outdated.
It's not one particular unit. Infact all early units have incredibly long life spans b/c of seige and bombardment. But this isn't a problem new to Civ 5. There has pretty much always been units in the mid game that obselete too soon in civ. Ancient units typivcally have long life spans (at least in civ 4 and 5 they do) due to seige.
Its not the legions/swords ballistas/cats that have the issue, its the crossbow/muskets/knights/trebs (mid game) type units that can have short to no life spans. As long as you can have a few long swords and a couple seige, you'll be fine and need no other units. I can't think of the last game where I built a musket. Why? B/c, one swords can be upgraded to longswords and are stronger and two, rifles are right around the corner.
Depending on your upgrades, rifle/infantry can have no life span. My jumps are usually either:
swords -> rifles -> mech inf or
longswords -> inf -> mech inf
either way you are skipping lots of units. I'd just prefer a slower progression and it have every unit actually matter or have a purpose in the game. Civ 4 had this issue as well where you can go from Axe/Swords right to rifles even on epic.
I used to play Civ 4 on Epic and could finish relatively peaceful games in 5-6 hours, war games in 8-9. But in Civ 5, I'm finding my regular games on standard approaching the 9-10 hours of game time level. Played one epic game, took waaaay too long and didn't even finish, so I can't imagine a marathon. I like to try and bang out a game in a week playing an hour or two a night. If it drags on too long, I lose interest and always want to start a new game and try something new.
In terms of overall pacing I find it annoying that (on emperor) its pretty consistant that Infantry come on line in like the 1700s. And that is by the AI and city states. I'm usually 1-2 techs behind depending on game. This isn't even with RA abuse or anything. Infact I don't sign RA, the AI does, but still when you aren't even trying to zip through the tech tree and you can get mech inf in the 1800's and build Apollo and stuff, then somthing needs to be done.
I just like my game to have an epic feel and right now the ages just move too quick. I don't see how on epic or marathon you still aren't seeing Mech inf in the 1800's or so, so I don't think the game speed is the issue. I just don't think there is enough to do in each age except advance to the next age.