Simple, do quests.
The main method of obtaining experience as a charismatic or intelectual character is to solve other's problems through the powers of persuasion, an army, or the glitter of gold. (Depending on your build of course.)
Sieges will be more difficult at the start but easy later on as your army size increases.
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Town and village quests can become quite tedious at times(the hunt for X bandits can take a while) but is a start. Always do the Peace quest when it's presented and you can afford it as it has quite a good reward considering you only need to find and convince 2 lords.
Village quests are fast and while the short term reward is small the long term reward of better quantity and better quality recruits will pay off in time.
Unfortunately there's no quick formula for a commander build, the start will always be a bit slow but the mid and late game will be most rewarding.
Depending on your character's morality you may find mercenary work for a faction to be quite profitable.
It's also a stepping stone into vassalage and eventual fief ownership which along with production enterprises will help pay for bigger armies which will lead to more fiefs through conquest.
Work hard to build up your relationship with the other vassals and the King, if the King likes you he will be more inclined to reward you with more fiefs. If the other vassals like you they will be more willing to follow your advice and vote for you concerning marshalship and the rewarding of fiefs.
Remeber, as a commander you're a thinker, not a fighter.
Who needs to fight when you can hire 40 knights to do it for you?