Staying in Tower of London area?

Its fine.

Tower Hill is the Underground Station closest to the Tower of London.

Length of time to get from Heathrow to Tower Hill will depend on how long it takes to get through immigration and which terminal you arrive at.

BTW if you refer to the Tube as the Subway you might get directions to a sandwich store.

Been traveling as a single female to London for decades. Have stayed in various neighborhoods. Never felt fearful walking around after dining or going to theatre.

If you wish to go further afield than the immediate area in the evening, you can take the No. 15 bus from the stop by the Tower of London west to Aldwych for Covent Garden, or stay on to Trafalgar Square. Or get off in front of St. Paul’s and cross the pedestrian-only Millennium Bridge to the Tate Modern and the shops and restaurants along the Thames Path on the south bank.

https://content.tfl.gov.uk/bus-route-maps/key-bus-routes-in-central-london.pdf

We like that area (St. Paul’s to Tower Bridge) for our London stays as we enjoy walking on the Thames Path. I would have no qualms about walking around this area if I were solo, especially in the summer when it is light until late in the evening.

I loved staying in this area. A friend and I stayed at the Thistle Hotel, very close to St. Katherine Docks, restaurants and the walk along the river. I arrived ahead of her and had no issues using the Tube from Heathrow. I did not use the express, just the tube. We also used bus 15 which is very convenient to go to other sites.

St. Katherine Docks is a fun area to wander around. You may even see the royal Barge, which docks there when h=not in use.

https://en.wikipedia.org/wiki/Gloriana_(barge)