Reader Question: Pacing the 1650

Every so often someone emails me with a swimming question.  Sometimes it's an easy question about finding a local pool or USMS team which I can just send a link away.  Sometimes the questions are all about me and I just reply with a quick answer.  But every so often I get an email like today that calls on me to ask for everyones collective experience.  MInd you I have my own ideas are will post a bit later but for now let's all help C out and find his/her pacing time for the 1650...

I was searching for some pacing information and one of your old posts came up from 2008.  I was wondering if you'd give me your 2 cents on a target pace?
Next month I'm swimming in a race for the first time in 25+ years- the 1650 - I basically haven't swum since then. I'm not trying to get faster, just trying to figure out what is a reasonable pace.
I swam 3x's with some of these results:
- 1,000 at a 1:30-1:32 pace - very manageable
- 8x 100 with about :20 rest at about 1:25 pace - still quite manageable
- a 600 a little under 1:30 pace
- several 50's at about :37 with :20 rest.  all very comfortable, but moderately hard
I'd like to do one workout next week that will tell me what kind of pace I can hit.  Do you think if I can do 12 x 100 at sub 1:25 with :20 seconds rest (without killing myself), then that might be a pace I can hold for 1650?
 I'm 48.
thanks for any advice!
C. 

In a follow up with me C writes back:

One other thing that is probably important is that I do compete at a high level in other sports in races that run about 20-23 minutes, pretty similar distance time-wise. so fitness-wise, I am in shape for this distance.  But I just don't know how to tell what is the magic pace that will not put me above AT in a swimming event, short of doing an all-out 1650 which I don't think is the right strategy.
thanks again!
c

Feel free to comment below and I'll pass it all along to C.  The power of social media and fitness.  Speaking of which soon I'll be linking to an article on PBS.org on the vert topic of people who use social media (ie. blogging) to achieve fitness goals.  I was lucky enough to be approached by PBS to be part of the article.  But for now let's all help C out.  Also since I have notice a lot of traffic in relation to the 1500 and the 1650 I've decided to put it in it's own category and over time including a series over the weekend will be posting more about these two events