I would rest, ice and see a physio ASAP. I ignored a groin twinge, trained for the London marathon through it, ignoring the increasing pain ('cos I'm a proper runner), turned last year's marathon 2.55 into 3.29 and finished in agony. Not currently running and don't think I will be for some time.
I am even older than you (just), and went sub 3 twice last year. I disagree with the comments about speed. I went sub 3 in 1998 and spent 15 years not doing it again! In my prep for my Autumn marathon (2.55.14) I raced a lot to get used to running faster, so sub 3 pace would be comfortable - weekly parkrun ideal. My long runs were typically 10 by myself and then the 2nd 10 with club mates at a faster pace. If you ran Halstead, I presume you are from Essex - plenty of good clubs.