now that i see an actual OP replying--even a year later--i figure you mean business. that's important. i'm 55 and sub-5, and while i come from a pretty good open pedigree in my youth, i still think it's possible for many, but requires a fairly serious commitment. my advice:
1) find teammates if possible.
2) set smaller benchmarks a month apart--could be any distance--and work towards them. prepare to readjust as you go. focus more on process than result.
3) have a weekly benchmark workout--could be a track workout that you do every other week to see progress, could be a tempo run that you do every week to see progress. be prepared, though, at times for two steps forward/one step back.
4) the side-effects of life are more real, and more prominent, with every year. as a h.s. coach, i used to be able to coach at an all-day track meet then run a race myself and do just fine when i was in my 40s. now, if i have a busy week of work i'm finding it affects my races more often. or maybe i'm evolving (devolving) into an excuse-making wu$$. or maybe a little of both. can't be sure.
5) since charlie just posted above, i figure i'll offer a suggestion i've taken from his book: eat a raw beet every day. i've had 18 months of pretty good health and fitness since i've done this.
6) do some fast strides/speed every day after your easy day runs. you can vary how you do it: my go to's are: 10x100 with every 3rd one fast; or, 3x150 "buildups" on a track, with first 50 jog/accelerate out of the turn to approximate mile race pace for 50, then last 50 hard, walk 150 back; or, 3x200 (200 jog), first one at mile pace, second at 800 pace, third at 400 pace. i suspect most of "the risk" older distance runners are afraid of stems more from lack of consistent speed work. personally, i've gotten injured more often from sporadic speed work, and less often from consistent daily speed. use it or lose it.
hope this helps,
cush