Fartlek — Swedish for 'speed play' — is an unstructured form of interval training in which a runner mixes faster bursts of varying length and intensity into an otherwise easy run, often by feel or using landmarks (run hard to that tree, ease off to the next). Flexible and fun, it builds speed and aerobic fitness while suiting the variable terrain of trail running especially well.
Interval training alternates repeated bouts of hard running with periods of easier recovery (jogging or walking). By spending time at high intensity — often near VO2 max — with recovery that lets you complete more hard work than one continuous effort would, intervals build speed, aerobic power, running economy, and the ability to handle and recover from hard surges.
A tempo run is a sustained effort at a 'comfortably hard' pace at or near your lactate threshold — typically held for around 20–40 minutes — that trains your body to clear lactate and run faster for longer. By raising the pace you can sustain before fatigue spikes, tempo runs are a key workout for improving race performance across distances.
The long run is the single longest run of a training week, run at an easy, conversational pace to build aerobic endurance, strengthen muscles and connective tissue, and accustom the body and mind to extended time on feet. It's the cornerstone workout for distance and trail runners, gradually increased over weeks and central to marathon and ultra preparation.
Chafing is painful skin irritation caused by repetitive rubbing — skin against skin, clothing, or gear — made worse by moisture (sweat), heat, salt, and friction over long efforts. Common spots for runners are the inner thighs, underarms, nipples, and where packs and waistbands sit. It's prevented with lubricants, anti-chafe balms, snug moisture-wicking clothing, and good gear fit.
A trail rating is a system for grading a trail's difficulty based on factors like distance, elevation gain, steepness, and how technical the terrain is. Ratings — whether official agency systems, color-coded scales, or app-based difficulty scores — help runners and hikers choose trails that match their fitness and skill, and set expectations for pace and effort.
Fell running is the traditional British and Irish sport of running over open, often pathless hill and mountain terrain ('fells'), involving steep climbs, rough descents, and self-reliant navigation in frequently harsh weather. Older than modern trail running, it prizes toughness, hill craft, and minimal support, and races range from short steep sprints to long mountain rounds.
A 100-miler is a 100-mile (about 161 km) ultramarathon, widely regarded as a benchmark achievement in trail and mountain running. Run over rugged terrain and through one or two nights, finishing one demands months of training, dialed-in fueling and hydration, crew and pacer support, and deep mental resilience. Famous examples include Western States and the UTMB.
A cutoff time is a deadline by which runners must reach a particular aid station or checkpoint (or finish the race) to be allowed to continue. Missing a cutoff results in being pulled from the race (a DNF). Cutoffs keep races safe and manageable, and in long ultras they force runners to manage pace, aid-station time, and fueling to stay ahead of the clock.
DNF stands for 'Did Not Finish' — when a runner starts a race but drops out before completing it, whether by choice or by missing a cutoff. Common in long and mountainous ultras, a DNF can result from injury, illness, stomach problems, exhaustion, or weather. It's a normal part of pushing limits in ultrarunning, and learning from a DNF is part of the sport.