Replacement for long bike ride

I don't really have much experience with this but was curious if there is a good way to approach it. I am set to do Maryland IM in October and have been training for a while now. I have my long bike rides on Friday's and I am scheduled to get in about 70 miles tomorrow. The problem is that it is supposed to be a total rain out Friday and Saturday. I typically have flexibility to ride either Friday or Saturday for weather purposes (and because I do my long run mid-week) but Saturday is supposed to be a rain out also and I have some prior commitments on both Saturday afternoon and Sunday.

The question is how do I replace the long outside ride. I do have a trainer set up at my apartment but I can't imagine going for 60-70 miles on the trainer as it is deathly boring and I just have a really hard time doing it. I have done 2 hour trainer rides before but it was brutal. Hopefully the weather magically clears up for a few hours tomorrow morning so I can at least get some outside riding done but what type of work should I be doing if I am unable to get outside all weekend? Is a long trainer ride my only option and if it is how much is enough?

Thanks all!