You might find that your Girlfriends stilts will get a bit softer for her if you bounce gently on them for a few hours don't go mad and bottom them just use them for a while so break them in
She will them find them a bit easier
The junior ones are not really a good idea for adults better to work on the ones she has
As for yours mate get out on them and use them you will build up the muscles you need to bend
A lot of people think that because they are fit in one way it works on all sports we once had a cyclist who did 30+ miles a day and more on the weekends (his legs were like Conan) and after 20 mins bocking he was close to tears because his legs hurt so much
So just get out and enjoy them
Jason