Europe inn-to-inn

Most of Europe belongs to the Schengen area, where one can freely travel from country to country, without pass control. As a US citizen, I can only spend 90 days during any 180 day period in the Schengen area. I regret that because some long-distance paths in Europe go on for thousands of miles and I would love to try a really long walk. Still, there is much one can do in 90 days in this paradise for inn-to-inn walkers.