Should I do it?

I have had a fantastic idea which would really help a friend with a project of hers, however it may take up quite a lot of my time and I am worried that my enthusiasm at the moment about this idea may wane and then I would be letting her down. 
I haven't told my friend about the idea yet, as quite frankly I am really confused.
What should I do Dragony Auntie?

Dear Philippa

Do you remember the story of Huck Finn painting the fence? He made it look so much fun his friends all joined in and ended up doing all the work!

Go for it, if your enthusiasm wanes then pass it on to someone else (after all maybe that's what your friend has done?)