I've played as an infantry this entire game but not purebred- I have a horse I use at the start of battles, because otherwise it's really impractical to set up your army well. I've been doing two-handed + longbow no shield all game and haven't found it that tough in large battles.
Some things that may or may not help:
Get the strongest armor you can for the least weight. You really, really, really don't want to get caught in a position where you can't get away. Sturgia has a really good armor that weighs nearly nothing, that one is helpful until you get your athletics up.
Don't overextend- ideally, you want the enemy not to be looking at you. Run in, kill a few people, run back out before you have a lot of attention on you. Don't get out where bowmen will shoot you, unless you're really comfortable dodging AI shots.
Watch out for cavalry. The quickest way to be incap'd is some lancer you didn't see running by from off your screen and hitting you in the head for 100.
When you do attack, get behind or to the side of the enemy (don't be in the middle of the pack) and use exclusively stabs and overhead attacks. Side attacks will almost always whiff on another troop.
Final pointer: if you want to level 2h skill, the bearded axe is common and wonderful. It has just the right balance of speed and reach to deal high damage at the most common melee distances, and also tends to cleave really well if you want to go ham on a pack of looters.
Hope any of that helps, good luck and enjoy! I know I have.