Don't wait until you get your bike up and running to get yourself moving. Any old bike will do, or you can swim, walk, run...just get some aerobic exercise and that will help with your ride.

And yes, you can certainly train for 100k in 8 weeks. There are some good training programs mentioned in these forums (use the Search feature). There are some for an English century in 8 weeks, and a metric century is really not so bad at all.

Just get moving.