Hey Polly...let me know if you hook up w/ a team. I will start looking for one soon.
I have done LOTS of MS 150's over the past 5 years. By spring I am normally in pretty OK biking shape. Running doesn't always translate to biking, but I have really good endurance. Slow but steady.
Amanda gives good info.
Here is my take and recommendation...WHATEVER you do to train DO a distance LONGER than 75 miles (or whatever the max distance is on one day). You NEVER know when things are tougher or longer than originally mapped out...or in my case when you end up on the Century route by accident. 2 years ago we had a team of newbies. Every weekend we would plan a ride. One day long and one day short. WE TRIED AND TRIED to encourge thes folks to ride w/ us. Most of them had never done 50 or 75 miles by the tmie we got to MS150 day. Yes, they survived, Had fun....but could of have more fun, etc. SO the next year we thought the would wise up and train better...NOT! They think we are "die hards" and even "hammer heads" (OK usually my avg speed in in the 15mph range unless drafting w/ a moderately faster group--and usually only hit 20 mph going down hill! or w/ tail wind--hammer head my arse!).
Time in the saddle...for tehe tush and privates--they just have to get used to it. And for the shoulders/neck...again time in the position makes it much more bareable.
And Amanda is right...you have time. And an indoor bike on a trainer is great for working on strength, etc. I felt much better prepared for spring riding by diong hard DVDs on the bike!
Good luck and let's stay in touch.
Right now I am training for a 1/2 marathon, so not much biking. But that will change soon enough!
K