The way to get to the Millwall ground is to get a train from London Bridge station to South Bermondsey station which is co-located with the ground and has a safe walkway from the station to the ground. The journey time is just four minutes and appropriate trains leave London Bridge at 14.01, 14.14, 14.31 and cutting it a bit fine 14.44. London Bridge is an odd station - platforms 1-9 are through platforms for trains coming from Charing Cross, Cannon Street and the Thameslink line that runs in between Bedford and Brighton. The trains to South Bermondsey, though, start from the higher numbered platforms for which London Bridge is the terminus - from what I recall they go from platform 11 though obviously you will need to check when you get to London Bridge.
But you obviously have four hours to play with after you arrive. Travel in London is covered by a series of concentric zones with 1 being the central area and 2 the next and so on. You can get unlimited off-peak travel which would mean all-day Saturday in zones 1-2 for £8.10 providing you have a bank card with you can tap in and out of stations - it simply stops charging you after you reach the limit. That approach is valid on tubes, buses and the local rail services including the the journey to the ground from London Bridge and services from Charing Cross to London Bridge.
Here are the details:
Covers Travelcards and Cap fares for Tube, DLR, London Overground, Elizabeth line and most National Rail services.
tfl.gov.uk
How you spend those four hours is up to you. A couple of suggestions are that you can travel from Euston on the tube down to Charing Cross on the Charing branch of the Northern Line tube and mooch around Trafalgar Square and maybe have a drink in the Chandos pub at the bottom of St Martin's Lane - you can see it from the Square - and then get a train from Charing Cross to London Bridge pretty well every train that leaves Charing Cross stops at London Bridge though do check and then get off there and move over to the terminus part of the station to get the train to the ground. Alternatively you can get to London Bridge earlier the quickest way is the City branch of the Northern Line tube - the Line splits in two south of Euston - and either go into the Borough Market area behind the cathedral on the other side of the road from the station or head down Borough High Street effectively the continuation of the bridge itself to the George pub - about two minutes walk away which has an outside yard which provided it isn't raining is a good place to have a beer.
Whatever you do - do not REPEAT DO NOT head further south than London Bridge on a through train. If you did you would have to get off at New Cross and walk back to the ground which is just not advisable.
Anyhow I rode past the ground on a train a couple of hours ago. It is empty at the moment but tomorrow afternoon let's hope it is a place of real excitement for us.