No panties under your bike shorts! This will just add chaffng problems. I am in agreement with Nanci that the jump in mileage is what's making your rear sore. Your bum just wasn't ready for that amount of time in the saddle. If you gradually add your mileage, your sit bones and everything else gets used to being on the bike, then the whole thing becomes much more pleasant. As for shorts, you may end up having to try a few different pair to see which ones you like the best. Length-wise is up to you, which ever ones are comfortable for you.