I've been cycling for ~9 months and have done 2500-3000 miles. I mostly ride in groups, and tend to ride in groups that are near the upper end of my range in terms of speed. The rides tend to be anywhere from 40 to 80 miles and usually have hills.
Do I have enough base miles to begin doing intervals? The mileage I've done has been at a fairly high exertion level (I'm usually well above 65-75% of Max HR), so I'm wondering if that means this mileage is not an effective base. Is there any harm in starting interval training, or should I consider logging some lower exertion miles before doing intervals and sprints?