I currently ride 9.8 miles one way to work 2 times a day for a total of almost 40 miles. The bike is a Magna, and mainly on dirt roads. As soon as I get a day off, I'm moving,(sucks, had the key to the place for 4 days now and just have no time or energy to move into my new place) making the 1 way commute 16.8 miles. Had the same problem with the seat, and was complaining to my buddy about it. He works nights at Chrysler and told me he had a seat for me. Came by after work the next day with a comfy one for me. Just couldnt figure out why it was in his lunchbox.... Anyways, it says on it "workmen cycles". maybe you could find it on the net. Comparable to the schwinn seat they sell at Meijers for $25, but IMO much more comfy! Hope this helps.
Mike