Get some mulching fabric from the garden centre (about 50p per metre) and cover the ground with it whilst you work out what to do first. It will suppress the weeds (if you leave it for 2 years they will die completely) and you can just pull back as necessary and dig over the ground you want to use without the weeds taking over everywhere.
Ask your allotment neighbours what the prevailing conditions are like, especially re weeds. We had couch grass in our plot and it turned out we needed to dig up and remove every tiny speck of root to get rid of it. If we hadn't we would have been fighting a constant losing battle.
Most of your work will be preparation now but it is good for your morale if you can clear a small patch and get something in like broad beans that will come up in the spring. That's what we found anyway.