I’ve just completed 8 weeks of Master’s base which is 2 weekly interval sessions (Tempo and Threshold) supplemented with 3 x Z2 outdoor rides. The Z2 rides are anything between 2 and 4 hours and I’ve been strict in keeping within Z2. This week I started Sustained power build but today I joined my buddies for a 2 hour Xmas Eve group ride, first one since starting base.
The ride wasn’t particularly fast but there was some dick swinging up the hills. I have to say I struggled up the hills with a very high heart rate and distinct lack of power in the legs. I don’t normally struggle with these sorts of rides so I’m wondering if this because of the base phase? And if so will things improve now I’ve moved into build?
Would appreciate any comments from people with similar experiences or who have more knowledge than me.
Base Training is as it says on the tin. Its simply building an aerobic base engine in order to begin the hard work of build and specialty. Stick with it and It’ll be them swinging their tiny d#cks way back down those hills 
Yup, it’s normal to lose a little top end in the base season. Efforts above threshold will feel a lot harder when you haven’t done any for 2 months.
Also, make sure you’re eating well and resting an appropriate amount. This will depend on the overall volume you’re doing, but high heart rate and a rise in RPE usually mean fatigue and/or illness is coming on for me.
I agree with what @MeMeToo and @jsiegrist both said.
Your fitness doesn’t exponentially increase all year long, and it’s normal for it to swing back and forth a bit depending on where you’re at in your training.
The image below is a basic representation of how your fitness isn’t always at a peak during training, but it should still be on an upward trend overall.
There’s some great relevant information in these two blog posts below. Check them out when you have time!