I get problems being in the saddle after about 50 miles or 3 hours or
thereabouts. "Aerobically" I feel I can do a ton but can't continue after
this perod of time - my max is 65 miles.

Sometimes it's a state of mind. Last year I was very comfortable with rides
of 50 - 60 miles and balked at the idea of 100miles. The turning point was
one glorious month where I did a four consecutive Audaxes at one per weekend
and moved through 2 x 100km to 1 x 150km and then 1 x 200km. The 150km was
just under an impewrial 100 and I added a few miles to top up after checking
in at the end of the ride. Having a ride buddy might be helpful, I
copuldn't have got through the 200km ride without the support of a fellow
Audax rider who was patient enough to wait for meon the hills

My thoughts are that I am just not
that used to going for this period on the bike and it will get better with
time. I hope so.

You'll get better. I have no doubt about that. I've made decent progress
over the past few years.

My position on the bike seems ok, my LBS says I look
fine. Maybe it's just the ache in the perenial hip bone area where my
weight (I'm a big 16st rider) is on that small area for a long time.

I'm a big 21.5 stone rider and have been lucky enough to be uncomfortable
only for the first few weeks of starting cycling again. It could be that
you need to have some short breaks during your rides as it's not to clear
whether you do the rides "in one" or break them up using a few refreshment
breaks. I'm not sure that what you describe is saddlesoreness - saddle
sores are due to chaffing and lead to broken skin, boils and sores. You
might need to experiment with saddles...

When I
lose wight AND get longer rides under my belt it'll get better? My saddle
seems dead horizontal to the ground. I used to have it leaning forward a
fraction but then I had numb hands after biking.

You haven't mentioned what sort of bike you have. You might want to
experiment with different hand positions on the bars. I rarely use the
drops on my tourer and road bike, using the hoods and the horizontal
sections for variety. Adding bar ends to flat bars might offer some
respite. Being lighter will undoubtedly reduce the pressure that the saddle
exerts on you.

Getting the position right is a black art !!

Getting the right saddle is even more arcane.....

Do my thoughts sound reasonalble guys?