I've just completed the final run of W4 and found it so much harder than W4 R1 and 2. Felt sick half way through and really had to push myself to finish it. Running on a treadmill at same pace as other runs so it's not because I went too fast. Hopefully just having an off day but at least I finished it. Now not sure whether to repeat this run or give W5 a go. Any advice please?