Museums take care of people with limited mobility. People in wheelchairs can skip the line. If you find you need a wheelchair during your visit, larger museums often have them available. And if the museum lacks a public elevator, they may have a service elevator you can use. Many of the most popular sights come with exhaustingly long lines that are easy to avoid if you make a reservation (good guidebooks explain how) or if you hire a private guide (who generally gets to go to the front).
Take full advantage of tours. Every town with tourism has a variety of tours that show you the sights from a comfortable seat. Orientation bus tours give you a 90-minute once over lightly. Longer tours usually do the orientation route with a visit to a couple of major sights (which involves some minimal walking). Hop-on, hop-off bus tours vie for your business in nearly every city.
They make a circuit lacing the city's top sights together and give you a ticket good for a day's worth of hopping on and off, with buses coming by several times an hour.
Nearly any company offering city tours will offer day trips out from a city, providing an easy way to see blockbuster sights on a joyride through the countryside. Every port city has a harbor cruise that gives visitors a relaxing and delightful angle on that town.
Cruise ships offer an array of on-shore excursions that generally include an option for those who don’t walk well.
Know your limits. You can opt out of that monastery on the hilltop and simply enjoy it from a café on the bank of the river below. And, if you're a good traveler, that café time can come with a memorable conversation with locals and an adventure in literally eating and drinking in the culture.
With the right approach and attitude, you'll find that because you move more slowly, you'll see a side of Europe that you may have missed on earlier trips. Consider yourself in the vanguard of the "slow travel" movement. It's a new world out there, and anyone with a sense of adventure can take advantage of all that Europe has to offer.
(Rick Steves (www.ricksteves.com) writes European travel guidebooks and hosts travel shows on public television and public radio. Email him at firstname.lastname@example.org and follow his blog on Facebook.)