I went on the Russia Revealed trip in Oct. 2011, and it's a wonderful trip! There is a lot of walking but probably not 4 hours straight. One of my travel companions had broken one of the small bones above the ankle about 3-4 weeks before the trip, and she had to wear one of those big boot type things, and she was able to do all of it. There were several people on the trip with canes, and one of the men had one of those sort-of walker things that actually has a little seat on it, so when we stopped for a bit as the tour guide explained something, he just sat and rested! And the suggestion about going to the restroom in the last spot before getting back on the bus to the riverboat was a very good one! The traffic was horrible, and it took us about 2 hours one day to go the equivalent of 14 miles!
We did the pre-trip to Helsinki and to Tallinn. Helsinki was nice, but it rained the two days we were there, and I liked Tallinn's old world charm a lot better....but my absolute favorite of all was St. Petersburg! Peterhof is an optional but don't miss it! The fountains, the gardens, and all is splendid.
A suggestion: we were forever tipping bus drivers and local guides (of course), and they will take US$$, so take lots of crisp $1 bills to use for that, or otherwise you will be wanting (needing?) to get lots more rubles all of the time---that's a pain.