Southend is a bit of a dive these days. I’d opt for Leigh-on-sea instead - it’s a few stops closer to London on the train. Old Leigh on a nice day with a pint of ale from the Billet and a pint of prawns is great. A few nice places to eat on Leigh Road too, plus a few B&Bs to stay in.
Epping Forest itself is lovely but not that huge so an afternoon walk and a trip to the oyster shack and attached pub is a lovely time. Waltham Abbey is nice to look at for a bit and there’s a couple of nice pubs near to it in the old town.
Depending on when you’re going, the Epping to Ongar railway will likely have an event on. The beer festivals in the summer are good fun.
A really lovely day trip from Stratford (or the line that goes out via Chelmsford) is to go to Manningtree on the train, and then do the circular walk around Constable Country (Flatford and Dedham Vale)
So much this, do not bother with Southend for staying in.
Old Leigh at least has a bit of character, and the other part has some decent places to eat (Stop the World was the last cafe I went to there, they had good cake)
epping forest is great for running. lots of decent trails. could prob do a nice 15-20 mile round route. cant say much about the surrounding towns but if you’ve got a car, epping is pretty easy to get to from the m11.
walden is really nice but not much to do. can probs see all the good stuff in a couple of hours. good place though if you wanna get to epping or cambridge. some nice runs round the local villages and countryside.
Another vote for Leigh. We occasionally go walking in Epping, but every time I’ve gone for a drink near there it’s full of geezers with one syllable names looking a tad leery.