Sounds like triathlon training? Even if its not, I bet they'd have training plans or advice for incorporating all three activities.
http://www.bikeforums.net/forumdisplay.php?f=191

Get a bike computer (check out the cateye strada cadence, ~$35-40?), then figure out what you want for a decent pace. Riding 3 times a week for 1-1.5 hours, it won't take you long to build up to 25 miles if you can already handle 15-18mph. Just be sure to include days for rest/recovery (harder to do if you're going to push yourself on the running/swimming as well as the cycling).