Lots. Unless you are an experienced mountaineer you will need to prepare for the physical and mental challenges of the Matterhorn well in advance. Rock climbing, scrambling and big days hill walking will all contribute majorly to this in terms of your fitness, agility, sure footedness, ‘head for heights’ and ability to know and manage your own kit efficiently. Training in the gym or in other cardio sports is better than nothing as supplement but in no sense as an alternative to mountaineering based training. The hills and crags of Snowdonia in North Wales and the North West Scottish Highlands provide superb opportunities for sustained days scrambling which will be excellent initial preperation once combined with some alpine and higher altitude training in the
week or 2 prior to the ascent. There are many alpine areas which give excellent mountaineering training suitable for preparing for the mountain. The most obvious two in terms of the volume, quality and accessibility of high altitude rocky alpine routes are the Swiss Valais and the Chamonix Valley.
Acclimatisation (and preparation) to try a technical four and a half thousand metre peak should be taken over at least a 5-6 day period, ideally with at least one other 4000m peak climbed and a night spent sleeping at around 3000m.