The comeback is possible - I've just completed mine. It's been a slow and steady progression beyond my wildest expectations, which originally were just to lose weight. I never even considered setting any new PRs, which were (at age 25):
800 - 1:49
1500 - 3:49
5K - 15:05
10K - 32:05
HM - 1:17
M - 3:11
At 38 with 3 kids, I weighed 205 lbs and had not even thought about running for 10+ years. Then I was inspired by a "master" friend who was winning our local road races. My starting point was a 22:30 5K in August 2013, which at the time was very encouraging because I didn't get completely dusted by my wife.
Flash forward. I'm now 41, 160 lbs and this have set PRs at every distance over 5K.
My journey started without a training plan. I basically hobby-jogged 2 slow miles every other day (~8 mpw). This allowed me to lose weight quickly while slowly build strength back up in my tendons, etc. Within 4 months, I lost 25 lbs and was running 20 mpw. My 5K time went to 20:00. Then I got a little more serious.
I added tempos and intervals on the track (ladders, mile repeats, etc) and gradually upped my mileage to about 40 mpw with regular strength training (P90X) and treadmill runs whenever my feet/legs hurt.
I vividly remember being amazed at finishing a 9 mile "easy" run at 7:45 pace. I sat in an ice bath afterward with feet/tendons screaming, drinking beers like I had just won a gold medal. I began reading LetsRun and was amazed at how much mileage people were putting in. In my era (90s) it was all about quality. I could never consistently run more than 30-40 mpw in college due to nagging injuries.
Six months into my comeback, I was 39, 170lbs, and had a breakthrough race - 10M in 58:40 which surpassed my goal of 60:00.
To make a long story short, I stayed very consistent in my training for 2 years, gradually upping my mileage to about 60 mpw. At 40, I was down to 16:30, 34:00, and 1:15, but started to stagnate. 6 months ago, I was struggling with anxiety and motivation. I dropped to 40 mpw and my times were slipping. Then I got a little more serious.
I purchased a training plan for the first time in my life ($30) from an elite coach. I picked one that matched my own training philosophies, held me accountable, and didn't leave me bored or struggling to coach myself.
For the last 16 weeks, I've trained harder (60-80 mpw) and smarter than ever before. In the process, I've set 4 lifetime PRs at age 41:
10K - 31:38
10M - 54:30
HM - 1:12:42
M - 2:46:07
As I've just completed the training plan which culminated in a marathon, I'm now taking a two week break from running to relax, assess, and figure out the next phase of training and life. I'm well aware that I may never get any faster, but am amazed at what has been proven possible with a couple years of consistency and dedication.
My apologies if this post has come across as humblebragging, but thought it was relevant to this thread.