First thing is not to worry, because this is NORMAL after a long   layoff. You work hard to build up lean muscle, and it disappears quickly   after you stop using it. When you come back, there is soreness from  the  tiny micro-tears and mini-spasms that dog returning runners after  the  hiatus.

The rule of thumb I got from a masters runner   who had been running competitively since college, was that it often   takes about twice as long to regain what you lost as it does to lose it.   In other words, 6 months off, plan a year for your comeback. This will   vary by individual, but the value of this rule is to allow time to   reverse the atrophy that takes place when your body reabsorbs the muscle   tissue it has no longer needed, and replaces it with weaker muscle  that  is more energy efficient at rest.

Christine's advice for   warming up is very important as you rebuild, because it will be much   easier to injure yourself during this time. Things to look out for as   your calves stiffen, is the dreaded "shin splint" on the front of your   calf, since the job of these muscles involves lengthening the stiff ones   on the other side. If you go at your recovery too hard, both sides of   the calf will stiffen from fighting each other, and it will feel like   you have legs of cement.

You will get advice to  stretch,  but remember stretching is a two-edged sword, which can help or  hinder  your progress depending on timing and intensity. Try to confine  your  calf stretches to using each muscle to stretch the other through  its  normal range and no further for a while, to encourage circulation  only.  Avoid using your hands or ropes, towels, etc., until your fitness  has  improved measurably. If you stretch against contracted (stiff)  muscle  fibers, the "eccentric contraction" can result in further tearing  of  the muscle.

Nutritionally, evidence is strong that a  high  protein meal shortly after your workouts, or before/during  depending  on the length of the workout, will be better absorbed and  utilized than  at other times of the day. While carbs are important for  muscle energy  and recovery, they are not your priority during your rehab  period and  can even interfere with the production of tissue-building hormones (this  is especially important for evening workouts). You'll need carbs more  as your fitness  and energy levels increase. Above all don't go hungry,  because your body  will break down more muscle tissue for protein and  energy if you do,  and your gains will be lost as they were in your  layoff. Timing is key.

There is so much bad advice in the  fitness world right  now, that it helps to get backup from authoritative  sources at times.  Doctors and researchers have spent lifetimes  accumulating valuable data  about how the body works, but this valuable  information often gathers  dust while entrepreneurs cash in on lucrative  pop-med fixes and  protocols. The following link to one of those rare  physicians who  specialize in muscle recovery, can explain your soreness  better...http://lifeafterpain.com/info/?s=post+exercise+soreness

I would do a walk/run interval for a good 5 min on a brisk pace to warm up.  Then after you warm up, i would stretch and then give your run a go.  I feel your going too fast before you even warm up and that is causing your tight muscles.  If you start slow and warm up you'll be able to do run your 8.0 on the treadmill without tight calves.  Just be sure your also hydrated before you begin your run as well as during and after.  If you slow down now you'll be able to avoid injury that would sideline you.  Nothing wrong with taking your time to build yourself back, as you said, Rome was not built in a day.

Do you wear heels a lot? I always find my calves get tighter with even a slight heel, so I try to wear flats on days I plan to run, and do some shin strenghtening/calve stretching throughout the day as well.

There can be several reasons for tight calves.  People often don't think about their ankles when they run (I didn't for over 20 years) but since I learned to not push off using the small muscles in my feet which would then engage the calf muscles and instead peel my foot off the ground and lift my ankles (not kicking or flicking your feet up as that will engage your hamstrings and you don't want to do that) I have not had any calf tightness.  Even after ultras my calves are no longer tight.  You want to keep your lower legs and feet and ankles really relaxed.  It is hard to hurt muscles that are relaxed.  You may be holding tension in your ankles - especially if you are new to treadmill running.  There could be too much "gripping" going on.  Check to see if your stride is too long. If it is, it will make you use more of your calf muscles as your foot leaves the ground.  How stiff are your shoes?  If they are not flexible they can cause you to engage your calf muscles needlessly.  I can give you more resources if you want.  I hope you are doing better!