How can I keep my horse fit over winter?

to spend more time in there. Cobwebs dust and old bedding can contribute to poor air quality. This will have a detrimental effect on your horses health particularly if they are already suffering from chronic respiratory disease. Cleaning and removing all the cobwebs can be a huge undertaking but will make a big difference to your horse. 
In addition to cleaning the stables you could consider ways to keep your horse occupied if they are going to be stabled for longer periods over winter. With increased hours spent stabled boredom is more likely. In extreme circumstances it can contribute to the development of stereotypical behaviours such as crib biting or health problems such as gastric ulceration. Enrichment toys mirrors or slow hay feeders can all help to alleviate boredom. 
Prepare and check your winter kit in advance. It is a good idea to go through winter rugs and have them cleaned and repaired if necessary. Check stocks in your first aid kit. And ensure you have a good supply of batteries for torches for those dark evening checks.
Exercise
Exercise has multiple benefits for both horse and rider particularly in terms of maintaining fitness muscle tone and increasing insulin sensitivity. Some consistent exercise is better than intermittent bouts particularly if your horse is likely to get fizzy or over excited if they have not been ridden which increases the risk of injury to one or both of you!
The main question is how to fit in regular exercise when the weather and daylight is against you.
In general an exercise program should be consistent and start off slowly with gradual incremental increases over time. It is not possible to recommend a one size fits all exercise program as each horse rider and circumstances are different. It is always advisable to consult your trainer a trusted experienced horseperson or your vet to try to come up with a realistic plan to meet your goals.
Hacking is a great way of keeping horses fit at any time of the year
It may form a large part of any training programme. Winter is a great time to get the OS maps out and see if you can find some new routes to try. Hacking is always better with friends and it is safer to have company especially on new routes. We always recommend downloading the What 3 words app which will provide emergency services with your exact location if the