Sintra is definitely not walkable in the way many people expect. Distances are short on a map but very steep, and moving between Pena, the Moorish Castle, Regaleira, and the National Palace can be tiring and time consuming.
Parking is possible but tricky. Lots are limited, often far from entrances, and you still end up walking uphill. The bus works, but it can be crowded and slow, especially in March if the weather is good.
What worked best for us was focusing on one main monument in the morning and then letting the rest of the day flow instead of trying to do everything. Regaleira and Pena are the most time and energy consuming. Trying to add the Moorish Castle on the same day involves a lot of stairs.
If walking is a concern, I’d avoid stacking Pena + Moorish Castle unless you’re very comfortable with steep paths. Regaleira + the National Palace is much easier to combine.
Sintra is much more enjoyable when you plan around terrain and crowds rather than just ticking boxes.
When I was there, I had a tour guide who made all the difference!