Here's my story for what it is worth. I ran in college on a not-very-good D1 team. PRs of 1:55 (800), 3:59 (1500), and 25:42 (XC 8k) (HS prs of 1:57 (800), 4:24 (1600), and 16:06 (XC 5k)). Never a high mileage guy -- probably peaked at 55-60 mpw and college track prs came off of about 45 mpw. Always got injured if I tried to sustain anything above about 50-55 mpw.
I more or less shut it down after college, though did some recreational running and the occasional road race (like maybe once per year) in the years following. Ran one decent race (1:18 half marathon) on very minimal training about three years out of college, and then was injured (actually during the race itself -- no surprise based on training). Went to grad school soon after and basically stopped running altogether, though joined the university's cycling team my last year and did a couple of races with the team.
At age 29 (soon after the birth of my first kid) I decided to run a marathon. Started running 10-20 mpw then did the 18 week build using the FIRST plan, which is basically low mileage, all quality. Probably peaked at 30 mpw, with 20 of that coming from a long run. A couple of months after turning 30, I ran the marathon in 2:54 (really poorly paced -- came though the half in like 1:21 and then kind of death-marched in). Felt good to be doing some structured training again though, and kind of whetted my appetite for racing again.
After some time off, I ran casually but consistently over the next year, averaging 15-20 mpw. Best race was probably a 5k in sub-17:10. Also did a few triathlons during this time. Tried to use the FIRST plan again for a spring marathon about 18 months after first, but got injured during the build (total case of trying to run too fast too soon off low mileage) and missed more than a month of training. Ran it anyway, in 3:06. Took some time off to heal, regrouped, and began consistently running 20 mpw or so during the summer. Had kid number 2. Built it up to 25 mpw (with one day of either tempo or fartlek and one longish run of 10-12 mi) by fall and ran a sub-16:30 5k and a 1:16 half off of that, at age 32. Also ran a ragnar-type relay with a bunch of my college teammates, which was super-fun. Ran reasonably consistently over the winter (15-20 mpw of just easy runs, no quality). Did first races of the year over past couple of months off that training, which went pretty well all things considered (17:00 5k and 55:00 15k).
Now 33 and planning to run a fall marathon. Currently running 25-30 mpw (with 12-14 mi long run, but no other quality) and planning to peak at 40 mpw (maybe 45) during build, using a Daniels-inspired plan. Goals for the fall are a sub-16:15 5k and a sub-2:45 marathon. Next year, I would like to maintain relatively higher mileage (35-40 mpw) but focus on the 5k and half, with the goal of breaking 16:00 and 1:15. In 2017, if all goes well, I'd like to take a shot at breaking 2:40 in the marathon. Basically, I am at the point where I am starting to chase my HS self (at least at 5k). By next fall, I'd like to be able to beat my HS self. Not sure if I'll ever beat my college self again, but we'll see.
To answer your specific questions:
1) How long did it take you to build your mileage back to a reasonable amount?
I'd say I'm just getting there now, so about three years. This is the first year I plan to run consistently more than 3 days per week.
2) Did you enjoy running by yourself?
Yes, that's never been a problem for me though I did just join a running club one of my college teammates is on and plan to do workouts with them a couple of times per month.
3) Was running ever the same?
Not the same, but still awesome. Really enjoying structured, goal-oriented training again and have enjoyed the self-coaching aspect (though obviously I have had some costly mess-ups). Racing is still super-fun. All of it is great "me-time" if you have kids.
4) What type of races did you enjoy doing the most?
So far it's been the 5ks when I've been in relatively good shape and felt like I was really racing for the win, as opposed to just out there running. The half marathon distance has also been fun, mostly because it's new and different. The two marathons I've done have been pretty brutal, but I'm still very much figuring out the event. The thing I'd really like to do but haven't yet is to run a road or track mile -- I have a feeling I'd really love it.
The other thing I did that was super-fun was the ragnar-type relay (12 people, 200 mi).
5) Was the comeback worth it?
Definitely. I think the key is being realistic about your time-horizon (i.e., think 5 yrs instead of 1) and making your goals realistic (i.e., chase your HS self as opposed to your college self).