Back legs and weight 9 year old springer

Chloe is a spriner spaniel though not very energetic she gets less food than my other dog 400g tin a day with a small amount of mixer I try to give her natural food and she gets her small treats in between, she is rather weighty and doesn’t seem to lose it however her back legs looks like she has arthritis she gets joint sticks and 2 joint tablets every day but sometimes she struggles to come or go up the stairs she has fatty lumps on her and one on her left back legs I don’t know how far to walk her incase she is in pain.