Beetroot. Eat beetroot.
Currently I ride about 38 miles a day but have an odd work pattern so cannot seem to get into a routine which is hard. Some times 9 days on the trott, some times 3 on 3 off, earlys and lates which mucks things up, my body clock and feeding. All I can say is try to eat well and get a full 8 hours sleep. Keep bike transmission clean and well lubed so as much of your effort is actually turning the wheels rather than making your bike sound like a cement mixer if the chain is totally dry due to rain having washed all the lube off which will take a lot of your energy big time. So re-lube, re-lube. This is what I hate so much about this crap crap summer we have had. The constant re-lubing as heavy rain strips the chain of it.
Also as others have suggested, ride a couple of days at 50% just lightly turning the pedals, it will only take you 5 mins more to get to work, honest, and will help you recover and build stamina.
Last week I rode two days the full distance 61 miles round trip door to work and back but the route is quite circuitous riding tracks and paths to avoid busy roads so I ended up quite knackered. I slept most of yesterday having stuffed my face at breakfast. Felt quite groggy in the evening but was ok today. Back to work tomorrow, early start, leaving at 6:14am.
When you get in try not to snack but boil a pan of pasta asap and drink lots of water while waiting. Eat lots of veg, salad, nuts, dried fruit, wholemeal bread, lean meat and rhubarb crumble and Ambrosia custard
. Obviously NOT all together. Try to eat a reasonable meal at lunchtime if this at all possible? It makes riding home a breeze. I am on my feet all day and it is rare to get to sit down except when riding to and from work. If the weather is really crap - drive / take train / bus. Don't be a masochist for the sake of it.
Try running several times a week just to loosen the leg muscles and restore bone density. Too much cycling to the exclusion of all other exercise is a great way to get osteoporosis.
HTH.