Simplify - ride more.
Doing CF style amraps and emoms are too short and explosive to build any kind of endurance for long rides. Great downhill fitness though.
Sorry Ajay but that is just simply wrong. HIIT has been proven to be very efficient at improving many of the physiological markers required for endurance. The programming, exercise selection and work:rest ratio may be wrong if you aren't seeing the benefits. AMRAP's & EMOM's are just fancy acronyms for different forms of HIIT. They are all just a period of 'Work' with a period of 'Rest/Recovery' in between. Stick to the basics, and gradually increase your work portion in relation to your rest and you will get fitter.
HIIT will improve VO2, maximise Excess Post-Exercise Oxygen Consumption (EPOC) causing increased calorie burning for 12-24hrs, maximally activate AMPK enzyme which increase mitochondrial size & quantity along with improving energy production (glucose uptake and fat burning) and concentric hypertrophy of the heart (increases the thickness of the heart muscle) which increases the stroke volume and the amount of oxygen that is transported to your muscles every beat. HIIT does all of this better than long slow rides.
So for the non-pros who are time crunched between cycling, family and work, there is a lot of value in HIIT during the week paired with some longer, lower intensity rides on the weekend.
Jellylegs, the key for you to improve your mountain biking is going to be improving your power-to-weight. Best way to make riding up hills easier is to carry 15-20kg less up them. Given you are struggling with hills and are 111kg, I'm going to guess that you have some excess body fat you could drop. Addressing this is going to be your quickest way to improving your cycling and the most efficient way to do this is through a combination of diet (as has been said already, eating good sources of carbohydrate, including protein in every meal/snack every 4hrs and getting good fats from eggs, avocados, fish, nuts etc is important. Cut down on processed sugar, fast food and alcohol at least 5-6 days p/week), HIIT and longer slower rides. Don't expect quick results either. Make the right improvements in lifestyle and just watch your weight and fitness results keep improving over 6, 8 & 12+ months. I would also recommend getting into the gym. Building up muscle strength, activation and mobility in the key areas will assist with injury prevention and metabolism. You will also look and feel better for it.
A basic form of HIIT is Tabata. You can do this on your stationary trainer. Tabata is 20 seconds of 100% effort, followed by 10 seconds of rest. Repeat that 8 times. Try warming up with an easy spin for 15 minutes and then do 1 set of tabata (4 minutes) followed by 4 minutes easy spinning before repeating for 1 more set of tabata (4 minutes) and then cool down for 5 minutes. That's under 40 minutes for a workout. Do that for a couple of weeks and then decrease the rest between sets to 3 minutes, then 2 minutes after 4 weeks. After 4 weeks, introduce a 3rd set of tabata into the workout. That's a very simple way of introducing HIIT into your program that is specific for cycling, is easily progressed, is time efficient and will get you results. Don't do more than 2 high intensity sessions per week though, you must let your body recover properly. Do an easy spin outdoors for 45-60 minutes on the day between HIIT's to aid recovery.
Also, stay away from running (especially at a high intensity) or skipping until it doesn't cause knee pain. Reducing your weight will help with the load on the joints as well. When you do start jogging, do it on grass. Jog one length of a footy field, walk back, jog a length etc. If the knees don't hurt afterwards, then do more lengths next time. Keep it simple and progress within pain free ranges/quantities.
Good luck.