I have been totally off the bike since september 2019 due to injuriy. Now i am fully fit but in a bad power shape 
I have made two sessions on Zwift and just uploaded the data to Xert. The TP seems a bit high and also the 5 minute power. I have literally not trained on a bike at all since september and only kept in shape with strength and some running. I expected a large drop in TP compared to what i just updated, so should i do a total re-sync or anything else to create a clean start?
A little sidenote is that i did a hilly Zwift race at the Volcano and made a 2,4 w/kg for the 29 km. Before my break i could have made the same race at about 2.9-3.0 w/kg.
Maybe someone else - more qualified - can give a better advise, but as for me, I started all over after my 3 months off the bike. Meaning, I deleted all data and started from scratch.
I know a lot of people consider the historical data of value - assuming it has been consistently correct, because some faulty power meter entries can frack up everything - but I personally do not care.
Now that I have a years’ worth of data, I never look back beyond three months, max.
I guess it also depends if you have an event coming in less than 3 - 4 months or not. I had one within 4 weeks of me getting back on the bike, so I had no hopes of getting any result. But, I did get a BT that day 
This I can relate to as my Training Load had been at zero up until October.
I’d set XERT to optimal decay and try for a breakthrough. If the signature look plausible I’d lock it in and start training. If you can’t make a breakthrough, go back one or more rides, go to Advanced MPA and enter in last season’s Peak Power and HIE. For Threshold Power enter a value lower than what you reckon it is now and hit save. From there XERT should be able work out your signature when it recalculates your breakthrough attempt. When first starting back riding it is better to underestimate than overestimate your TP and LT. The act of using last season’s PP & HIE values will aid this. Your current PP and HIE will come to light as time passes.
Training wise, I’d set it to Triathlon and start building up your base with lots of rides under LT. XATA doesn’t have a big range of short endurance rides in its library so you can always take XATA’s recommendation when it comes to the required XSS but meet it by riding at 0.8 or 0.9 of LT until the target XSS is achieved. It will be hard to keep your Form off red/tired to begin with but it will get easier as your training load increases.
Your fitness should improve rapidly so you may want to schedule a breakthrough attempt every few weeks.
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