On the road, you really do need at least 10mins of very slow warm-up pace, otherwise you get tired out quickly, as you describe. Each run becomes a "pace" session, so it is hard to build up endurance.
When you start off, go at a shuffle, a snail's pace, so slowly that briskly-walking 80yr-olds would pass you, so it feels like running in slow motion, with only slightly more effort than sitting in a comfy chair at home...find a mental chant that works for you -
I use "slow and easy, slow and easy..."
- after this compulsory 10min shuffle, you can pick up the pace a bit -
For this phase, I use "nice and steady, nice and steady..." to set the rhythm.
- but Slaphead is right, pacing aint easy.