Well, it depends on what you are trying to accomplish. I live in Central FL so I gotta go drive to find a decent hill.
I think with 1000 miles in your legs, you can do intervals. You warm up. You get yourself up to your top cruising speed and then you go pretty much as hard as you can. You can hold that for 1 minute up to three minutes. You back off, recover and repeat.
People also do hill intervals. Probably the best way to do those is to go and find a nice steep rather short hill and climb it until you are done.
If you do these things right, your legs will be cooked. So your next day's ride might be a recovery ride or a long slow distance (LSD).
Muscles can be very specific in function. If you do hill intervals, they probably will not help your flat ground power so much and vice versa. You might alternate them.
Also, if you are doing intervals intensely enough, you should probably only do them once per week.
Intervals are good for increasing your top end power.