You might find that building a positive mental image of you completeing a race in a relaxed and happy way (or wasted but victorious - whatever works) and really thinking about it, making it rich in detail etc. really helps with the motivation.
As everyone else says, find time, even if it's scraps of time. Like parking a couple of miles from your brother's house and running there and back, just try and work in those runs.
I've entered the London Duathlon in the 'ultra' distance class - 20/80/10, I'm not aiming for a place, just to enjoy the experience and not humiliate myself.
I've done a fair bit of running (10ks to marathons) and riding (a few races etc) - all mainly offroad.
So with 15 weeks or so to go I thought I should start structuring the training plan a bit better.
I've got about 10-12 hours a week to play with, but with some restrictions, mainly in terms of duration of a session and location - 2-3 of these hours will need to be indoor at home. Probably only 1 4hr+ session is possible a week.
I was planning to do the following:-
Long run - 12-15 miles - 1 evening a week
Long Ride/brick session - building towards 80k (but I live in hilly Surrey so it might be less), 1weekend morning - I'm thinking to just tack a short run on the end of the ride to get used to the transition.
Tempo run - 1 morning a week (1 hr @ threshold)
interval ride - 1 evening a week (1 hr turbo)
Core / Strength session 1-2 sessions a week
If anyone has any better suggestions I'm all ears.
Also - should I go for a tri-suit or just normal cycling shorts - given the duration of the ride?