Blimey, 50 is hardly old, with luck you should have years of walking ahead of you.
Attempting any Long Distance Path, especially one as long as the Pennine Way, needs time and longterm preparation.
By what you are saying, you seem to be struggling with long distance fitness.
Attempting a large section of the Pennine Way, when one is not used to long distance walking, is not advisable.
Now if you said you were 70, and suffering with painful feet, then i would understand your concern.
Obviously you do not want to disappoint your son, but the Pennine Way will be there in the future, and being only 50, aim for the walk in the near future.
Over thirty years ago, i could walk 50+miles a day, over tough terrain, and hunger for more.
Even though i am still extremely fit, and approaching my 60th birthday very soon, i could not cope with such a distance, simply because my legs are no longer used to walking that far.
I run regularly every week, still walk distances of 20+miles most months, but i would struggle on the Pennine Way, simple because i no longer do challenge walks, that involve time on your legs, for mile after mile, on a weekly basis.
I can easily cope with 20miles, but getting up the next day, and doing it again, and again, i would eventually call it a day.
Remember, you do not have to wild camp any sections of the Pennine Way, as its such a popular walk accommodation for most of its length is readily available.
Wild camping is more an adventure, but getting out into the bleak Pennines knowing there's a warm bead waiting every evening, has its benefits.
Build up to it, not jump head first, and even though a lot of the route is straight forward, you still have to get accustomed to walking long distances, day after day.
That can only be achieved with longterm practice.
A walk as long as the Pennine Way is meant to be savoured, not dreaded.
Good luck with the training.