2-3 hour walking tour in Lisbon

Download the Rick Steve’s Europe audio app. There’s the Lisbon city tour you can do for about 2 hours on your own. Download it before you leave For Portugal. Free.

or just get his book and use those tours. Hard to tell which to do.

Alfama was a lot of uphill walking.

Belem area is a little further out; has a great monument, Belem tower and the amazing St. Jeronimo's monastery.

Baixa was all level;
Bairro Alto was probably more interesting after Alfama - depends what you want to see and do...
I'd take an Uber. The airport is 4 miles from downtown. Takes about 20 minutes.

feel free to contact me.

If the timing works with your schedule, I thought the Inside Lisbon "Best of Lisbon Walk" 3 hours (18Euro in 2019) was excellent. It was recommended in the RS book.

Otherwise, as others mention, the RS book has self-guided walking tours for different parts of the city.