Change in plans

It's looking like Paris-Brest-Paris is going to be a no-go for me. I'm learning the hard way that at about the 300km mark my back begins to give out. 3ookm is only 1/4 of P-B-P so I can't imagine my back holding up to that. I still plan to do the rest of the shorter brevets this season and am thinking about new challenges. Perhaps I'll do the STP single day on my soon to arrive fixed gear? I'm not crazy about the crowd at STP so perhaps I'll do the ride solo on another date? Also thinking about a tour from Vancouver to Astoria on the Adventure Cycling route. That could be a good 4 day tour. There's also riding from Seattle to Vancouver on the fixed gear. The nice thing about a new bike is that it opens up new challenges.