Hi there,
Unfortunately I don't know exactly where Boulder Aikikai is--I got into Aikido after I moved away--but I know you certainly aren't going to be able to catch a train there. Public transit in the Denver area is still...(searching for a charitable word here) rudimentary. You might be able to get an RTD bus, but I think that's your only chance w/o renting a car or catching a cab. LoDo is quite a way from Boulder.
Best of luck, and have fun out there! Make sure to snag a green chile beer at the Wynkoop...
Cheers