Ok i am wanting to start focusing on getting better speed and power in my rides.

So i started intervals today.

problem was when on flats i have a hard time to motivate to go hard enough.

but if i find a reasonable hill. spot. I can hammer up the hill and get my heartrate much higher and sweat

Would it be ok to just do this type of training with hills only, vs not doing 100% on flats?

What do you suggest? or say for better motivation?

i wasnt exactly timing my intervals. I was more less going really hard between like say a sign on a road to specific lamp post, etc

for at least 3 minutes at at time.

Thanks

On hills or on the flat, I suggest you time them. It's easy to overestimate the time you are spending at close to maximum effort.

It doesn't much matter whether you do them on hills or the flat as far as boosting your fitness is concerned. Obviously if you are out of the saddle a lot of the time you use different muscle groups, so use a mix of standing and seated.

I agree with timing but less so from the bio/physical aspect......time those runs like a personal TT to guage progress. Hills can keep you motivated because it's a fight you dont want to give up on easily. Simply put, when you start a hill you dont want to quit. On the flats your motivation can drift as you said.

Dont give up on the flats, extend your distances after you have progressed on those hills.

Most people don't ride a high enough cadence when attempting intervals on the flat. You want to be in the 100-110 region. If you've warmed up well, and can pedal hard at over 100 cadence, you shouldn't have any trouble getting your HR up. That said, you may need to practice the fundamentals before doing intervals. IME, you should have at least 1000 miles of fundamentals first. Meaning practice with one-legged pedaling, long intervals first in zone 2 and later zone 3 at cadences over 115, these intervals lasting over 1/2 hr. Plus regular multi-hour long steady distance rides of at least 60 miles. Plus long moderate effort hill rides. Then try long seated hill climbs at 50 cadence. Stuff like that. Learn to pedal perfectly first, then learn to pedal hard while maintaining form. Then you're ready for intervals. Don't rush it. Improving at biking is a multi-year effort.