Never do long duration exercises eg. an hour on the treadmill / bike. As a trainer I find there's no need for it and they usually leads to loss of focus, sometimes injury. Interval training and weight-training
has better effect on aerobic ability. I'd say forget the long, slow exercises and stick to the short, sharp workouts. Intervals are the only realistic option for burning fat, increasing speed and stamina, and developing functional fitness. Minimum three times a week.
Consistency is the key to obtaining great results. Even if you feel a little tired just train at a lower intensity on that day. It all adds up. Don't break a good habit!
If you struggle finding the energy to even get to the gym, one nutrient can help alot with that slumpy feeling.
Here comes the science bit: Co-Enzyme Q10 is a nutrient that is required for electron transport, generating energy within cells. Q10 is found primarily in meat and fish but it is also found in spinach, broccoli and whole grains.
Essential fatty acids Omega 3,6 & 9 help with fat loss as well as heart, brain and joint health. Sources are fresh fish like herring and mackrel as well as seeds such as pumpkin and sunflower which can be sprinkled on cereal / porridge. Can also be taken in capsules.
Eat five moderate sized meals a day and ensure a serving of lean protein eg. fish, chicken breast, eggs with veg at each meal. Eat food as fresh as possible (meaning as close to it's natural state, not packaged), and cook by steaming or poaching
to retain the nutrients. Also drink plenty of water.
Hope some of that's helpfull if you didn't know it already. Good luck, keep us posted!