depending on how long your commute is and how often you do it you might just need time to make that amount of riding become easier.
when i first started my 13 mile each way commute i was BEAT. i slept like 10 hours a night most nights and generally just felt worked over.
over the course of a year or two the commute just became part of the routine and i was able to commute reglarly and still enjoy some mtb riding on the weekends.
over the last 1/2 year i have actually found i became faster when i cut down my commuting overall and focused on riding harder a couple days a week and just commuting on my "off" days.
so don't underestimate how tired a new longish commute will make you until your body adapts to it! but commuting (especially forcing yourself out on the days you really don't feel like it) is a great training tool for long rides. it may not be the best training physically but it is awesome for the mental side of the game.
Last edited by max-a-mill; 06-26-12 at 01:29 PM.