11 year old border collie with 4/6 heart murmur

Recently my vet said Skittles my border collie has a 4/6 heart murmur. She's been put on Hills Science H/D diet. She is weighs a shocking 68 pounds and should only weigh 45-50 but the vet didn't say anything about her being overweight or how much of the H/D dry food and H/D canned food to give my dog. About a year ago Skittles tore a ligament in her back right knee and could hardly walk for 4 to six weeks (as a puppy Skittles got a partially torn knee ligament in her back left knee after our other dog charged right into her and sent her rolling down a hill). For almost a year Skittles has become quite a bit less active and has gained 10 pounds. I now wonder if she has become less active because of her bad knees or because she was becoming easily fatigued due to having a heart murmur.  We live in a two story house and Skittles has a really hard time going up the stairs.  I thought that was also because of her bad knees.  But maybe it is just too hard on her.  Skittles will stand in front of the stairs and start to go up one stair and then come back down and start over again often several times, before she finally gets past the first step and then goes very slowly up the stairs, and sometimes she stumbles a bit. Should I block the stairs? Is it too hard on her heart to let her go up?  Have you any recommendations for appropriate exercises for her?  She likes to tug and she likes to have the frisbie thrown.  I have been making sure that I don't throw the frisbe so that she has to jump for it and I don't throw it very far so that she will just kind of canter-skip after it.  Is it alright to keep these activities up?  I love Skittles with all my heart and will be very diligent in doing whatever I can to help her.  Any kind of advice that you can give me about her diet, medicine and /or suppliments, exercise/fun time activities and anything else that you can think of for her betterment would be greatly appreciated by me.