This is more difficult than it seems to be at first glance. I will try answering it, though.
It has been a long time since = good. We are talking about sometime in the past until now.
I had a chance to attend a morning...= I would say "have had." My thought is that we are talking about an experience--a nonspecific time in the past
Think of another sentence : It has been a long time since I have eaten
The rest is okay, and I think it was the above part that was confusing.
Anyone else have any different ideas?