Interesting, I still think I'm in the newbie territory, although my body is adapting relatively quickly and so far my heart is responding pretty fast. The plan over the next month is a lot of endurance rides and keep my HR in the same zone.
Also dialing in (aka having smaller) breakfast before I do my workout on bike has helped a lot. Riding on a full stomach doesn't agree with me.
A small breakfast and lots of goodies in the pockets is the way to go!