My 85 lb Collie is going on 15 years old........?

I've had this dog for the last 13 years and got her when she was 1-2 years old, the vet leaned more towards 2.We didn't know since I got her from the death chambers at the pound.Literally they were walking her into that room.She means so much to me and I realize that eventually she will die, however just this year everyone, my family and friends, keep telling me I need to start thinking about her burial or cremation since she is to big to bury in the back yard.....She seems perfectly healthy to me, she's still as spry as she was when I got her, no eye problems, no teeth problems, no coat problems, I realize she's getting old but she still looks and acts fine.......Why does everyone keep telling me this? And how much does it cost to have a pet cremated or buried in one of those pet cemetaries? Is it something that has to be arranged in advance?And how do I tell them and get them to listen that I'm tired of them telling me these things?I've already told them it depresses me.