Definitely agree with Islandplodder re doing walks of similar day lengths and on hills. Age doesn't matter as long as you prepare for it. I'm 58 and we walked the C2C 6 years ago, but we did the Pennine Way last year. Training for them was definitely a key factor to completing them, we've done shorter, easier long distance paths that I didn't necessarily find easier as I hadn't done so much training for them. We live in a flat part of the country so had to have a few weekends away on the hills in preparation. I walked just with my husband not in a group, though he is fitter than me, but pretty good at walking to my speed!
Another couple of tips - make sure your walking boots are worn in and get used to the rucksack that you will be carrying (fully loaded). As you are doing a guided walk, I imagine you will have your luggage transferred each day, but you will still need a day pack for your lunch, water , waterproofs etc.
How many days are you walking it over? You've still got plenty of time to train up for it. Also, might be worth posting this in the Long distance walks section as you may get more replies there.