The website for that hike has a great page on the required fitness:
The minimum capability is:
You should be able to walk 3h00 steeply uphill, followed by a 3h00 descent on rocky ground, whilst carrying a rucksack of around 10+ kilos. You need to be at ease scrambling, walking on scree and large blocks. Below are some relevant uk walks.
Then it lists several UK hikes that help you evaluate an individuals fitness:
- Ascending Snowdon by Grib Gock.
- Descending from Blencathra in the lakes by Sharp Edge.
- Scrambling along the Aonach Eagach ridge in Glen Coe.
These are excellent day hikes and not only will you show the kind of exertion the big hike will take daily, but those of you with more experience will be able to refine your equipment and skills.
As such I recommend you plan for at least two required "shake out" hikes before the big trip, planned close together (maybe one week apart) to give you enough time to fix equipment problems between, but not so much time that the difficulty and aches and pains from the previous hike will be completely gone and forgotten. Make sure everyone's pack weighs at least 10kilos.
Make it clear to the group that if, by the second hike, it appears someone is having a hard time dealing with the effort that they should withdraw and plan on training for a to be determined big hike a few years later. There's no shame in withdrawing, but there would be a big problem if someone couldn't maintain the group's pace during the smaller day hike.
Be open and honest, and accept that someone might get their feelings hurt - that's far better than having to split up during the big hike, medical issues, or even having to end the big hike early.
If she doesn't bow out herself, make some backup plans. Create a plan that has you at certain locations by certain times, and if she can't keep up create plans for evacuation for them at points along the trail. If they need to rest for a day, continue on indicating that you intend to keep to the schedule.
Alternately, make a plan for them to join you for only a day or two at a time during portions of the hike, rather than the entire journey. Have them book nearby hotels and sightseeing for the days they aren't hiking with you.
