The rule of thumb is to take equal or slightly less recovery, so lets say you run 800meter in 3mins you would than take upto 3mins to recover, intervals are not about speed but more v02max which takes around two mins to reach and done at about 3-4kpace, so for 3min 800meter you would spend 1min at v02max on each intervals, the longer the interval or shorter the recovery the more time you can rank up at v02max.I put it into prospective for you, for 6x800meter with 3min recovery 18mins worth of hard running you would spend about 6mins at v02max infact you can pretty much guarantee 6 mins at v02max even if you are fully recovered before the next interval, yes i know my example is not ideal but it gives you the idea.
I've notice my heart rate is higher when it's hot, today i went on a what was meant to be an easy 4 miles at around 156bpm but ended up as a steady run at 166bpm, The heat does get to you and the heart beats faster and the veins dilate to get the blood around faster to try and cool you down, you will also sweat more and if you arnt well hydrated will also make the heart work harder, so in the heat make sure you are well but not over hydrated and take it easy even tho you will be going slightly slower to normal, but your heart will still be in the correct zone.
If you only run 4 times a week than you can only fit one quality session a week, maybe two at a push, by quality i mean intervals,reps,hill reps,tempo and long runs. all the rest is just easy runs to recover and to reach weekly mileage. You long run should be 25-30% of weekly mileage, intervals at no more than 8%,reps/hill reps at 6%, tempo run of 20-45mins or 3-6miles or 10% of weekly mileage depending an what you want to achive.Going back to how many times you should run well try and run 5 times a week and include two quality runs with one should be a tempo/cruise tempo run. long run is not the main workout for a 10k runner, maybe for a Half marathon or marathon but not a 10k,
day3 warm up, 3x1 mile (or 3x10min with 2min easy) at desired 10k pace or 30min at marathon pace aka tempo run
day4 rest or same as day 1
day5 35min (4miles) light light fartlek (speed play) or if race/5k park run on day 6 do same as day 1
day6 rest or same as day1 or race/5k park run, if race on day 7 just 30min easy or rest.
day7 40min easy or 6 miles or race or if race on day 6 just rest.
about 16 miles or 2.5 hours minimum a week, you can do more if you desire but as a beginner i'll stay around 15-20miles a week untill you feel that you can up the mileage on each run or do an extra run a week.