Consider and remember the uncounted multitudes of fathers over centuries of time who have watched children leave....including your own father. Doesn't make things any easier... not a bit. But, you aren't alone. I still sit in my daughter or son's room once in a while. A million memories of family times endearing and beckoning.
The kids are out there, riding the wave of maturity and independence throughout their lives to come that you helped create. Chances are, they'll raise their kids in much the same way you raised them.
Better to have had them than not, better for them to be in the world though physically distant, better to go call one of them just to say "Hi"...they're carrying part of you with them always as you do them.
Then, good to go for a ride and dedicate that next hill to what you and your kids have given and will continue to give to each other...whether at home or "out there"-- but always close in your heart and theirs.