If you have never done any kind of interval training before then almost any sort will benefit your fitness.
Threshold interval training is useful (for example) for helping you when you need to get on the back of a passing faster group, or indeed working as part of a rotating group where you may be threshold on the front then drop to tempo or even endurance heart rate levels once sheltered by others. Also useful to get yourself over a long drag of a hill without dropping too much speed.
You could start with something as simple as this on your 10km section. Go as hard as you can, for as long as you can. Then when it feels like you need to slow down, do some easier pedalling till you feel your heart rate has recovered. Once heart rate and breathing back down, go hard again, until you need to take a breather, then repeat. You can wear a HRM to get an idea of how hard you were working, but you do not need to look at it during the efforts. These are a bit more unstructured compared to fixed time intervals, but if you recover quickly will allow you to go harder sooner, and if you are recovering slower, will naturally spread the harder efforts further apart.
Then your long rides can be just that, long rides to work on your efficiency over distance.