Running on grass is SO good! Especially when its not summer because its slightly squishy and leaves a small imprint of your foot... I used to run on concrete all the time... no wonder I got a sore back from jarring! After running 30 mins on grass I went onto cement to get to my car and there is just no give in concrete whatsoever. I hurt a lot more after grass running but thats cos the muscles in your legs adjust to the sometimes uneven surface - but it strengthens the legs far far more and yeah its way better for you. Just thought I'd let y'all know 
Also in terms of HIIT... I ran on a rugby field.. I did a few laps for 10 mins to warm up and then I did HIIT in lenghts rather than minutes/seconds... it absolutely WASTES you. I couldn't manage it for much more than 10 mins. I find it easier because you don't have to keep checking the clock to make sure its exactly 30 or 45 seconds or a minute... just do one length then turn around and go back. The ppl training there may have thought I was nuts, but hey I was having fun
Also in terms of HIIT... I ran on a rugby field.. I did a few laps for 10 mins to warm up and then I did HIIT in lenghts rather than minutes/seconds... it absolutely WASTES you. I couldn't manage it for much more than 10 mins. I find it easier because you don't have to keep checking the clock to make sure its exactly 30 or 45 seconds or a minute... just do one length then turn around and go back. The ppl training there may have thought I was nuts, but hey I was having fun