well numbing, no matter when it starts (15min or 1.5 hours) is never
a good thing.
First thing I'd check is the saddle itself. If your saddle is tilted such that
it's putting alot of pressure on your perinium that could be the cause.
A small change in saddle angle can make a lot of difference.
You also might consider having LBS help set up saddle for you (and
do a fitting which may help with this problem).
The next thing I'd do if the angle thing doesn't help is look at a
different saddle. There are lots of different saddles out there, a lot of
them are anatomic in that they have cutout down the center (or depression)
to relieve pressure, The other alternative is something like a Brooks B17 which
is wide enough to support your sit bones (if your saddle isn't wide enough to
support your sit bones your soft tissue will carry all the weight).
Alot of shops have bins of take off saddles for not a lot of money. You can
always try the hit or miss method, and if you're getting numb after 15 minutes
a quick test ride should answer that question.
Wallingford Bicycle parts sells
Brooks saddles with a 6 month unconditional guarantee, if you don't like it
return it.
By the way, after trying cut out saddles, gel saddles, the only one that I find
comfortable is the Brooks B17, and no numbness.

Hope this helps,
Marty