Work it into your regular activities . . .
Last year I was an 8-to-5er and to train for my first sprint tri I biked to work, about 11 miles round trip, 3-4 times a week. Twice a week I went to the pool before work, about a 10 minute ride from home. I found that I could get into the pool by 6:30, swim for half an hour, shower, then back on the bike by 7:20 to ride to work (mostly downhill!), clean up and change and in my cube by 8 am. Went for an occasional run after work or on weekends, as I was planning to run-walk the run portion.
I have no idea how people work full time and prep for longer distance races, but would love to hear about it!