Lou, I'm a beginner runner too. There are a few of us, and other more advanced types that also chat and advise each other in the exercise forum, so check this out.
Yes you can do other exercises on your rest-days, but it's advisable to avoid exercises that pound the joints in your legs, at least until you've built up strength in your bones and joints. Cycling is supposed to be an excellent exercise for runners because it builds up strength and endurance in similar muscles without putting too much strain on them

(however, running is not supposed to be as good for cyclists, wierd I know).
Also check out the coolrunning website for lots of information and advice, don't know the URL, but you'll find it if you google it.
Happy running

and good luck at the weigh-in tonight