Echo the feelings of the posters here.
Me and Mrs G had to take our greyhound to get 'the jab' a few years ago. We took him to the vet with a bit of a limp- figured it was prickle in the paw or something. Xray later showed an osteosarcoma (bone cancer) in his hind leg. We were given the option of euthanising or keeping him for a few weeks/months until it became unbearable.
We took him home with some painkillers. He showed no change for 4 months or so, still running around like a mad boy, climbing stairs etc. then one day just refused to get up out of the sandpit. Within 12 hours he'd gone from walking/eating etc to virtually nothing. We knew it was time but it was pretty late in the night. He was breathing slowly and had a consigned look on his face.

We rugged him up for the night not expecting him to make it through the night. Eventually got to bed. Woke up early the next morning and he was still going

. Rang the vet and got an appointment jammed in between xisting bookings.
Had to lift him on a sleeping bag through the house and into the boot of the fairmont. Folded the rear seats down and had Mrs G laying in there with him. (Not real safe but wasnt thinking about our well being at that stage). Vet came out and helped us lift him into the room. He asked if we wanted to be with him- I expected Mrs G not to want to be there but she surprised me. We were with him through out. A weird feeling, was obviously upset as he slid away but also relieved in a strange way- I swear he gave us a 'thank you' look as he left....At that stage it was the hardest thing I had to do in my life.
Shit that made me upset posting that...take it easy Moe