Are speed sessions worthwhile if not very advanced runner?

Whether you're advanced on not, it helps to do both longer aerobic runs and speed sessions. Also, medium distance tempo runs help, too. It doesn't matter if you're advanced on not. In fact, the best way for a beginning runner to become a more advanced runners, is to do all of these types of runs. However, if you're new, work them in gradually, and give adequate rest days in between them, until you're used to how it affects your body and recovery.

Effective training should include a variety of paces/efforts, with each specific session planned for a specific reason.

It depends how un-advanced you're saying you are. If someone was in the first few weeks of couch to 5K I wouldn't advise for them to do speed work. Also, there are different types of speedwork. You could just do a few strides at the end of your run. Or 8 x 150m flat out. The former is better than the latter if you haven't been running long. I'd start speed work by just introducing small things.

Before doing speed work I used to do lots and lots of mileage including stupidly long runs and I could barely run 19 minutes for 5k. Cut the mileage back to 45, did faster stuff and took 3 minutes off that time and ran sub 80 half.

Let them do their long slow runs man. I'm happy to beat them in races.

Those are the ones who would benefit the most.

Twenty years ago, I joined the Jim Spivey Running Club. It was a new group started by Jim. He designed running plans for those who wanted to be coached and the rest of us met for a once a week interval workout on the track. The members were 35 to 50 years old (me) and were mostly 20 to 24 minute 5k runners. The workouts varied each week. Mostly we did ladders of 200m to 800m, 200m reps, 400m reps, 250/150, 200m with a cut across the track jog between reps.

At our two year anniversary, I did a survey of improvements in our members and found that 41 of our 43 members had set PRs in their chosen race distance since starting the weekly interval workouts. The two who did not set PRs had nonrunning injuries that set them back.

So yes, adding a weekly interval workout will help you become a faster runner.

how fast do they have to be relatively speaking? how long is the break?

You sound like a retired South African cricketer.

Yes ... and at the very least, speed sessions improve your running economy/efficiency.

Do 5 year olds like to run fast or slow? The answer to that question pretty much says it all. Everybody can benefit from speed work. However, if you have no run done any fast running in a while, you need to be smat.

The three key things to remeber

1. Warm up propeely. And does not mean run two miles slow. You may need a mile slow to get your blood flowing and your muscles warm. In summertime, even half a milr caan be enough. You then have to stretch, then do some lighter drills to preload the muscles to be used explosively. And it a slow progression where the first drill is just going thru the motions with not too much explosiveness, but eventually getting mor explosive. Then a few short sprints 6om to 150m. The first one, you are just going thru the actions more than pushing off the ground hard. Fronm the second one, the trick to always do these as buildups, where you are not going close to full tilt until you are 50m in. And don't stay at full tilt ehen you get to it, just let the body casually hold as much as the speed without trying to do any more acceleration, and then allow the body to taper down without stopping suddenly.

2. In the actual workout, take as much recovery as needed. For the first few times you do these workouts. Don't try to start cutting rest until at least the third week. For some in poor speed shape maybe not till week seven. And remember the key pointer from the first point about not trying to keep acceleratting. Just one acceleration, then relax and hold that speed as well as possible. Doing more than one acceleration is advanced work. The key is relaxation.while running fast. Trying to accelerate a second time on that repeat can and will get you injured. Be smart. Even well trained athletes get injured trying to make that second shift. A good example was Mboma in Kenya when she saw how far Shelly Ann Fraser had gotten ahead of her. That second shift got her injured.

3. Dont't do too much to start. Don't worry you will get there. Be sure to warm down when done. Light slow stretching after working out will help recovery. Remember, as a beginner, it will take a minimum of 48hrs recovery before setting up another speed session. If you are still sore 60 hrs later, it means you did too much the first day.