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.
Bit slower than I wanted (1.28), a PW, but the injured leg held up, and a modest increase in pace in the last uphill section. Original plan of sub 2.50 looks some way away at present.... Might settle for 2.59.59!
BTW queuing for a loo this morning and once I got the front of the queue, some woman with a clipboard told it was reserved for some charity. Clearly unaware of what charity means.
Regarding the starts, I really wouldn't get exercised by which start you go from. I've done all 3 and I couldn't say hand on heart it made a jot of difference.