In political discourse, the terms radical and extreme do often seem to be used in such a way that the only difference between them concerns degree and connotation, i.e. that a person or an ideology which is regarded as radical is just a little bit less extreme than someone or something which is considered extreme! But there might be more distinctive semantic differences than that.
Etymologically, radical comes from the Greek word radix, which means root. Extreme, on the other hand, comes from the French word extreme, which in turn comes from the Latin word extremus, and means, yet at this day, outermost or farthest. Extreme can thus be understood as something which is the opposite of what is considered normal and average. What is regarded normal and average can, however, change quite largely over both time and space. Today it is normal to be more or less obese, at least if one lives in North America, Oceania and Western Europe; and in for instance the US’ states with the largest share of obese people, such as in Mississippi, a fitness lifestyle is indeed extreme in relation to that particular average body constitution and general outlook on dietary normality which are to be found there.
Cause these things are mostly about nutrition. Even though the majority of relatively serious fitness athletes work out pretty hard and do that something like five or six times a week, it is the amount of calories that is the key divide between X (or I) and A (or O) shaped body types, not the number of dumbbell lifts or interval sprints. Hence, obese people need to be nutritionally radical in order to change their lifestyles and, consequently, their physiques.
In Sweden, there is TV show called Du blir vad du äter (You are, or become, what you eat), and the host of the show does often empty people’s fridges and make them start over from scratch next the time they visit the local supermarket. She thus pulls the root out in order to let something more constructive grow at its vacant place. Sometimes people have to do things like that; when slight modifications simply are not enough.
Radical changes do sometimes overlap extreme behavior, both in a good or in a bad way. Generally, though, to be extreme is of course not to be considered as good, since a proper balance between for instance reading, writing, eating, training and sleeping are required to develop as a person, intellectually and physically. Extreme behavior within the fitness domain is perhaps also associated with the use of illegal substances, which is likewise not to be encouraged.
But I think that it might be just as important to deconstruct and reinterpret the meanings of normal and average as the likewise context-dependent meanings of extreme and radical. If normal and average imply bad health, or at least something less constructive, than to be radical and/or extreme – in relation to that particular average and normality – then one should gladly embrace these things in the proper sense.