combine RS "the City" walk with St. Paul's Cathedral tour?

No it is not too much and a good plan.

If you like walking you'll be fine.

Have you also looked at London Walks? They have wonderful guided walks all over London. Lots of rave reviews on this board.

I agree with MaryPat - a good combination as long as you are in reasonably good condition and have happy feet. I'd do it.

As was once said by a very smart man, "When a man is tired of London he is tired of life" (gender neutral of course these days - he was before that)

As already stated, they can be done on the same day with no problem. Plus, they are both in the same area of London. You could do half the city walk, visit St. Paul’s, then finish the city walk to avoid having to backtrack.