Yes halfs are great because you're not dead feeling for 3-4 weeks after. I can train for a half in 8-10 weeks and do pretty well with my time and recovery is only a week or less for me. But effective marathon training takes me 13-16 weeks depending on where i'm starting from with my long runs.

Ultras---I want to do a 50k someday and eventually run the Comrades ultra in South Africa (56 miles I think?). That'll have to be later in life when I'm not working so much.