Did that when I was 19. From the posts above, I gather the old
route is Timpohon. That was what I took at the time. Basically, the
route, at least as far as to the laban rate resthouse, is pretty
straightforward and involves nothing more complicated than climbing
steps. However, i will not say it is very easy. I recall it as
being pretty darn exhausting, though I was very unfit at the
time.
The way my friends and I did it was to travel to the resthouse in
time for some dinner and a bath. Early next am, we then tried for
the summit. This was definitely tougher than the last section.
There was no real steps here and you simply followed a guide rope
up to the summit.
You can descent in a day. however, be careful. I think the descent
is more dangerous than the ascent.
Edit: Did some googling and found this website: http://www.borneo.com.au/Mesilau.htm
The following quote ought to prove helpful to you
The Mesilau climb does involve a climb up, then a drop into the
Mesilau valley, and then another climb to regain the lost height.
That was a great arrangement, as we got to see both routes. Our
guide arranged transport (30 RM) to take us back to our car at
Mesilau when we reached park HQ. The drive is about 30 mins.
Overall, if you've climbed the mt before, I'd recommend the Mesilau
route. If not, then the traditional route is probably just as good.
If you like to meet other westerners, then the traditional route is
definitely better! Pretty jungle the Mesilau way.
Hard to compare distances, but I'd say it should be only about an
extra hour the Mesilau way.
In summary, the old route is probably easier for someone who
calls him/herself 'undertall'. 
