Back in 2005 (about half way through my PhD) I took a week off to cycle more than 500 miles from Edinburgh to Brighton.
Although physically it's one of the toughest things I've done, it was also one of the most enjoyable. There was something about the simplicity of the task that made it strangely relaxing; all I had to do was get up in the morning, start cycling south, try not to get lost and try to eat enough. All other concerns were secondary.
But finishing was strangely anti-climactic. I was happy not to have to sit on a saddle again for a while, but the achievement also meant the loss of the goal.
I felt the same after finishing my PhD. It felt good to submit, but it also left a big hole to fill.
Goals aren't necessarily about the outcome, they are about the process. And the reward doesn't come after you finish; it comes in the form of meaning and focus in the present moment.