fwiw, i find that the fitter i get, the longer it takes me to warm up. given that 8 miles = <30 minutes, that's a pretty typical warm up, no?

also, you dont provide any information about what you're actually doing for training in terms of volume, intensity, what is a "quick" group ride, how long are the hills you're repeating, what do you mean by "intervals".

just based on the tone of your question, i suspect you're new to riding, racing, training for racing, and are making typical noob errors. these typically are solo rides have lots of tempo or terrain based intensity riding that are too easy for real improvement but too hard to sustain and do your other workouts at optimal intensity, that your other stuff has lots of anaerobic work capacity intensity followed by low active recovery intensity/coasting. the big gap in that method of training is that you arent really doing anything that can sustainably develop your aerobic capacity (or functional threshold power).

oh, to answer your question, i really doubt overtrained, but rather under-rested. it's really hard to over train. it's not very hard to under rest.