Anaerobic training is designed to develop your anaerobic capacity to exercise. It does not matter what form it takes as long as you achieve that objective. It so happens that intervals/repetitions would be one of the most convenient ways to do it.
Individual strengths and weaknesses should always be considered but the rule of thumb would be that you should NEVER jump into drastically different type of exercise. If you do most of your conditioning on relatively flat area and never had much chance to expose yourself to hilly courses, you should start out the hill training phase very carefully. Same thing; I don't know what type of hill training you did, or how you did them, or what type(s) of training you've been doing over the years; I would not recommend jumping into doing 200s or 400s, certainly not at 5k race pace.
Coming off what you've been doing, think of what you have developed best and what you have not; you have been doing lots of aerobic running at relatively slower speed; so you have endurance but not speed. Having considered that, I like what Ron Daws would suggest best. Do longer repetitions at easier pace, if you want, with shorter recovery. I like ladder here by doing, say, 2X200, 2X400, 2X800, 2X400, 2X200...something like that. You can move up to 1200 if you want. You're not used to doing faster training so starting out with only 200s is good psychologically. In the original Run to the Top, if you analyze it, he would start out 20X200 at 1/4 effort, then 20X400 at 1/4 effort, then 20X200 at 1/2 effort...so on and so forth. Always building up to a little bit more and little bit faster. Same thing. Don't just jump into certain quantity and certain pace. Work your way upwards, not downward. You still have plenty of time to run faster still. Just be patient.