Whilst I completed LEJOG after my retirement, I did it in stages LE > Minehead > Chirk > Edale > Kirk Yetholm > Milngavie > Inverness > JOG. Not the straightest line but it kept me off road and on National Trails for a good way. In answer to your question re legs, having walked the 630 mile SWCP in one go my legs were absolutely fine. By the end I had high muscle definition especially the thighs and front calf muscles. Feet (toes and heels) needed attention but Compeed kept me going without too much trouble.
The things to take care of fairly obviously are your "mental state", take one day at a time. Don't think of all the miles ahead of you, reduce the weight of your rucksack whilst ensuring that you walk "carefully" so that your hips don't take too much pressure - don't jump from one rock to another!
Mike age 71!