I love this thread.
I'm 36, about 6'3" tall and 195 lbs with no extra weight. Almost 20 years ago, and 30 lbs lighter, in high school, I ran sub 18:00 routinely for 5k, around 39:00 for 10k; and just over an hour for 10 miles. Typical highschool XC training. No pain, no gain. Repeat miles. 12x400 in 72 seconds with 200 m recovery jogs or until you puke. Fun stuff.
Fast forward to 2002 where I started riding mountain bikes seriously and running occassionally (mostly during winter) to maintain fitness. In late August '07, a friend talked me into a duathlon which was on October 14. I hadn't been doing much running, but I had been riding single-speed mountain bikes 5-6 days/week for well over a year, so I had some decent fitness.
In September, I started running three days a week, and continued riding mountain bikes on the other three days- short easy runs (3-4 miles max, good warmup and cooldown), with a heart rate monitor, staying at or below MAF. September 29, I entered a small local 5k (first road race in 16 years) to test my running fitness and ran just over 22 minutes on a flat course. I went out far too fast, running the first mile in 6:38 and suffering through the remainder of the race, but with my limited training this got me wondering if, with increased mileage and a little more structured training, a sub 20:00 5k was in my future.
On October 14, I ran the 2.2 mile runs in the duathlon (my first) at about 7:00 pace (with a BIG hill in the middle), and finished in 22nd place out of 80 (I am strong on the bike). The second run was brutal, but my pacing was consistant even though my legs felt like lead after the transition from the bike.
At any rate, I've continued doing 90% of my runs at or below MAF, and running 3-4 days a week. My MAF pace has dropped by almost two minutes in 8 weeks. Everything feels good. I'm working on increasing my mileage to about 25 miles per week (in 4 running days). I recently added a weekly tempo run, and my current tempo pace, as of today, at 86-87% MHR, is about 6:55. There is a small local 5k in two weeks, and I'm wondering if sub 21:00 might be possible?
I'll be adding more running over the winter, as cycling becomes less accessible. As spring approaches, I'll start on some real speed work. I'm going sub 20 in the spring for sure! My long-term goal is to try and get back to sub 19:00 by next summer!