Hey, girl,
I'm a chronic lurker as well, but I read your journal and just had to chime in. I, too, ran my booty off (literally!!) in my late teens. . .and quit running altogether after a failed marathon attempt. . .at mile 15 I figured out I had the flu. 104 degree fever, the whole bit. After that day, I didn't run a step, or really exercize meaninfully for (the past) 7 years. I have the exact same fear of failure after such success that you have expressed, but about 6 months ago, I summoned up the courage to join the WISH board and walked a bit. Then, walked and jogged with WAY more walking than jogging. Now I'm going to the Y for water aerobics and walking on the TM, but am about 60 pounds heavier than I was when I was running 5Ks, 10Ks, 1/2s, and the full. I was an overweight (registered "Clydesdale" runner), even at my thinnest and everyone was always so supportive. They didn't see me as a fat girl, but as an athlete. But, oh well, because I'm about 19 down from my heaviest and life is just a journey full of ups and downs (with weight, emotions, Lane Bryant sizes, everything!). Its not about constant improvement or feeling afraid to regress a bit. Its about making the effort during the journey and about how you feel about yourself at any given moment.
You can climb back into an athlete's lifestyle. . .in just one workout! Take each day one at a time, or one week's worth of workouts at a time. . .just like your 5K distance, but these sentiments are especially true for longer distances, trust me. As you build up you must not panic that maybe you aren't in shape enough, just relax and let yourself just be.