You will be fine going into London @Muchtoomuchtodo, you just need to make sure you get a train which is going at least to Kings Cross Station, because some will end their journey's at Earls Court. The tube station you will want to get off at, as I'm sure you know, is Russell Square.
Going back to Hounslow you will need to check that the westbound tube is going to Heathrow, not Rayners Lane or Uxbridge as these are after the branches split. If you do get on an Uxbridge train by mistake, don't panic, you can get off at Acton and then change to a Heathrow train. In fact, it can be better to do this as there may be more Heathrow trains going through Acton than at Russell Square. The train is also likely to be less busy by the time it gets to Acton.
Plan a journey - Transport for London This says Hounslow West to Russell Square is about 47 minutes.
London is the best city in the world (yes, I'm biased). Unlike our reputation, Londoners can be friendly and if you get lost, or confused as to whether you are in the right place, please ask a TFL employee or another traveller. A lot of travellers may be semi-lost tourists, but lots will be experts on the tube system and be able to help.