Chunking Full Swing Shorter Irons 7-Gap

[Gimmemopars]

1st Newbie question.  I had back surgery earlier this year and am now semi-retired and I've been playing 2 to 3 times a week for the last 2 months.  I recently bought some 2016 Taylormade Mx2's and have had some mystifying results.  When I first started I left the 4I at home because I was never able to hit one, but low and behold, in the first few rounds I did hit a couple of 6s and 5s flush and was amazed how nice they flew, 125 to 140 yds, which is pretty good for me.  I'm pretty stiff in my swing due to the back and being overweight doesn't help I'm sure.  

Well I brought the 4I out and was hitting beauties over 150 regularly.  I can't tell you what it was like to see little grass pieces in the arrangement of dimple marks on the sweet spot of my 4 iron!  I was also hitting very nice half swing pitches with my PW and GW from 30 to 40 yards as well as nice chips with 7 on up to th GW.

The problem I'm having is that I can't seem to hit full swing shots with my 7 to my GW, I almost always get too much turf and thank it.  My new clubs are an inch longer than my old standard clubs so I'm guessing that may be part of the problem, but I hadn't used the old clubs for almost a year because of the back surgery, so it's not like there was some built in memory there.  I had the lie checked and everything is up to mfg. spec.  BTW, as you've probably guessed, my favorite irons used to be my 7 thru PW with my old set.  Used to slice or shank the longer ones.

What can I do?  Look at standing farther away?  Won't that affect the lie angle?  I just don't understand how I'm cleanly getting the 4, 5 and 6 and sometimes the 7 to go and I can't get a full 8 to GW to go cleanly.