Well, according to the AA, it's a shade over 300 miles:
Roots Hall to Victoria Park avoiding motorways
As a cyclist, you can assume that you'll be able to manage 12 m.p.h. - but let's be conservative and say 10 m.p.h. (makes the numbers easier).
That would represent a raw figure of 30 hours' worth of cycling.
You should then factor in that you need probably a 15 minutes stop every hour - for a breather, a wee etc., and 4 longer stops during a days' cycling - an hour for lunch, an hour for tea, a coffee break etc.
So, factoring in fitness levels, breaks, meals, sleep and things like punctures, you'll be doing well if you cycle for more than 6-7 hours in a day. So, let's presume you'll travel 70 miles a day, then it should take you somewhere between 4 and 5 days to get to Hartlepool from Southend.
Oh, and it'll take between 4 and 5 days to get back, obviously.
So long as you set off first thing on Tuesday morning and put a big day in, then you should theoretically be in Pools in time for a 3pm kick-off on Saturday; coming back, you should be back by some time on Thursday the following week.
That said, out of an abundance of caution, I'd clear two weeks in your diary - leave Southend on the Monday, and bank on getting back by the Friday after the game.
All part of the friendly SZ advisory service, you know...
Matt