Congrats!  I turn 50 next year, but started to train for a 5K after foot injuries made me sedentary ~ fat ~ out of shape.  I ran my first race last Saturday (Girls on the Run) and so can tell you now the things I wish I'd known.

a) walk or practice run the course first. Know where the challenges are, know how you will overcome them. For me, big hills always intimidate me. I like to know where they are so I can plan accordingly. We're running a Turkey Trot Thanksgiving morning and I'll have run the course twice, this builds confidence...

b) food - in the morning before I had a Lara Bar (coconut :-) and a double shot of espresso (kick, but not too much fluid).

c) gear - Don't wear wide-legged pants if it is raining.  Now I know why someone suggested crop, tight running pants. You get wet in the wide leg stuff, well, it sticks to you, and stretches down, and it is all pretty miserable. 

d) iPod...need i say more?

e) be PROUD. I beat myself up for two days because my last run was better than the race run. Oh well. I still did something I'd never done, and I did it through injury.  Finally, I let it go and realized I was still awesome, fitter, faster and lighter than I'd been. 

You GO! 

Wow -- and excellent run.  And I think that you were holding back!

Next time -- sub 10 minute miles.

Wow, congratulations on your first race!  I came into this thread late but it sounds like you have a ton to proud of!!  50lbs and a 50 min five miler, that's awesome!  50 yrs old was my magic year that kind of kick started my running and health commitment also and I've been doing it for nearly 4 yrs now.  Like I said I came into this thread late but I totally agree with Vol's advice that you should run and race for the enjoyment and pleasure of your accomplishments first and foremost and those accomplishments should only be measured against your own goals and expectations.  Running and races are so much fun and the people are great no matter how fast or how old or how far you run, it's just a great community of people that share a very fun and satisfying sport.  Welcome to the fun and congratulations on your awesome accomplishments!!!

Pete