There are various public office routes by which a character can become eligible to command an army or fleet:
Former Legate -> Legate
Consul or Former Consul -> Legate
Praetor -> Former Praetor -> Legate
Pontifex Maximus -> Former Pontifex Maximus -> Censor -> Former Censor -> Legate
Appointing a new praetor at regular intervals is probably the easiest way to get a stream of new legates.
Also, every now and again you'll get a message from the senate offering to appoint a character to a leaderless army, so you could always accept even if you don't get a very good commander.