I agree. Walking from the Hilton to the old city center (Centro) is not an option (during the day). The Hilton is located in Bocagrande almost as far as you can get in Bocagrande. The distance is too long and it is too humid for walking. At night time it may be a pleasant walk (but still humid in my experience from last month in Cartagena). However, Uber works well and there are plenty of cheap taxi (look for the small yellow cars - no meter so agree on the fare first) and if you really want to save money, there are frequent buses (look for the green/colorful buses) going from near the Hilton the city center (as I recall it the fare is COP 2,400 - to be paid in cash to the driver when you enter) - I took the busses numerous times when staying at the Hampton (which is also a nice property and the location is much better/closer to Centro - almost walking distance). There is also the Transcaribe bus T103 between Bocagrade and centro, but that requires you to have purchased a card in advance and loaded it with money - you can't pay in cash to the driver.

2-3 days in Cartagena is enough. Too many tourists and hassle from street vendors - and the beaches are poor. If you go for the beaches, Santa Marta should be your choice.