I would suggest you buy a decent heart rate monitor. When you find you are running at a lower heart rate for the effort you are exerting, THEN your body is ready to move to a harder effort if you want. A harder effort might mean a slightly faster time, a longer distance, speed intervals. You should not do ALL of these harder efforts during the same run.
I have been running now for 22 years. I am 64 and have run over 250 races of a marathon or longer. Last year I only finished one out of 7 attempts of my favorite distance race (trail 100-mile ultras). So this winter on my treadmill every day I picked 2 days for running speed intervals, something I had not done in several years because up until last year I COULD finish 100s yet. I also quit eating desserts and have lost 6-8 pounds of body fat over the winter. I'm hoping an old body CAN learn new tricks! For me at my age and training style history 5.4 miles/hour flat on my treadmill was the pace I started at in late November, after both donating blood and my last race. I did that every other day and on alternating days I walked at 12% grade (my treadmill maximum) at 3.2 mph. My heart rate for 30 minutes to 2 hours averaged between 106-110 bpm for both of these slope/speed combinations. After 3 weeks my heart rate had recovered a bit from my blood donation and I was feeling a little stronger having recovered from having run 3 marathons, a 100k and 68 miles of the Bear 100 mile race in the previous 6 weeks. Yeah, I was exhausted (totally). Then in mid-December on Tuesdays I started with running 8 mph flat for 0.1 mile and then 5 mph flat recovery for 0.1 mile and I would repeat that for 5-6 miles. I would start these intervals after an initial 0.5 mile warm-up wherein I would start walking flat at 3 mph and every 0.025 miles increase my speed by 0.1 mph till reaching 5 mph. My heart rate the first 8 mph segment spiked to 125 bpm and by 5-6 such miles it spiked at 143 bpm. On Wednesdays I would do intervals of the same 0.1 mile lengths at 12% slope only running 5 mph (fast) and walking 3 mph (recovery) and I would did this for 3-4 miles. My heart rate climbing would reach a couple bpm higher and get down a couple bpm lower due to the longer times at these speeds. After 4 weeks of this I saw my heart rate spikes/peak getting 3-5 bpm LOWER and the the speed sessions "felt" easier. I then lengthened the fast segments to 0.15 miles while keeping the recovery sessions at 0.1 miles. The first week I did that it was hard. After 4 weeks it felt better and my heart rate spikes/peak had dropped a little and i went to 0.2 miles at high speed (still 8 mph flat and 5 mph at 12% slope). After 4 weeks that felt better, but I was only 3 weeks out from my first planned 100-mile race of the year so I started to cut back on my distance each day and keept the speed of each session the same. On Thursdays I (in December) ran steady pace flat starting at 5.4 mph for 5-7 miles and every 4 weeks increased it by 0.1 mph. On Fridays I walked steady pace for 3-4 miles at 3.2 mph at 12% slope. On Saturdays I ran 12-18 miles flat at 5.4 mph. On Sundays I walked 7-10 miles at 12% slope and 3.2 mph. On Mondays I ran 5-6 flat miles at 5.4 mph. Each week that I increased the length of my speed segments I also increased by 0.1 mph the speed of all my other run runs and walks. I ended up having to stay home and not run my first intended race of the year (Umstead 100 mile run) due to a family emergency, but my second race is this Friday, so I just maintained my same speeds and distances for another week, except that yesterday I was able to run a shortened (3 mile) flat speed session wherein I ran from 2.4 to 2.9 miles at 8 mph and my heart rate only reached 137 bpm. This shows how much improvement my body has achieved over the course of this winter by using some speed sessions each week with a 4 week time constant between increasing efforts. I felt like I could have easily gone further also at 8 mph this past Tuesday, but with a 100 coming up this Friday I did not want to chance it. I will not run now until start of race.
I think if you study what I described, you will discover a "system" varying your running schedule on a weekly basis of hard and easy efforts each day or each couple days (never doing more hard efforts (distance or speed) more than 2 days in a row). For beginners, I do not think doing "hard" sessions more than one day hard and 2-3 days easy is appropriate. What is hard and what is easy is UP TO YOU!!! But using a heart rate monitor and SEEING what YOUR heart rate is when your effort feels hard versus when if feels easy will go a LONG way in helping you define a running plan that will fit your goals.
Get fit, stay fit, run long, enjoy life - it is the only one you will ever have!