What's happening with this Time Machine Backup?

I bought a Seagate Backup Plus Slim 2TB on Tuesday (10/13). Initiated a Time Machine backup of my iMac (24- inch, Early 2008) / 2.8 GHz Intel Core 2 Duo / 5 Gb 800 MHz DDR2 SDRAM (upgraded RAM myself some yrs back - 2 slots 4 & 1) / El Capitan 10.11.6 / 89.05 GB free of 319.21 GB on Tuesday night round 9:30 PM EST. The intent is to migrate everything to my mid 2015/MBP 15-inch/Retina.

Now the dilemma: The backup finished at some point over night, and was at 8% "Encrypting..." when I woke at 6 AM Wednesday morning (10/14). Wednesday evening (round 25%), I turned off Time Machine to see what would happen. Nothing happened. So I turned it back on. It is now 9:45 PM, Thursday night (10/15) and it is still Encrypting... at 60%.

Should it really take what will be about 4-5 days to complete this process? That's crazy!!! Did I "break" anything by turning TM off? It doesn't look like that caused any hiccups... Is this just because it's an old system?

Lastly, here is the storage breakdown if it helps in anyway:

320 GB SATA Disk / Audio 48.11 GB / Movies 21.18 GB / Apps 10.76 GB / Photos 6.87 GB / Backups 45.6 MB / Other 143.18 GB

The only app I really used on it was Logic Pro 9. At this point, I'm just letting it ride out, but wanna know what gives. I've disabled sleep, screensavers, anything that would make it stop. And, I have a small table fan blowing at the air vent on the upper back of the screen helping cool it round the clock.

*scene*